Introduction to
Materials
Management
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Introduction
to Materials
Management
Eighth Edition
Stephen N. Chapman, Ph.D., CFPIM,
North Carolina State University
J. R. Tony Arnold, CFPIM, CIRM
Ann K. Gatewood, CFPIM, CIRM, CSCP
Gatewood Associates, LLC
Lloyd M. Clive, CFPIM
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ContEnts
Preface
ix
1 Introduction to Materials Management
1
Introduction 1 / Operating Environment 1 / The Supply Chain Concept 4 / What is Materials
Management? 9 / Summary 13 / Key Terms 14 / Questions 14 / Problems 15 / Case
Study 1.1: Fran’s Flowers 15
2 Production Planning System
18
Introduction 18 / Manufacturing Planning and Control System 19 / Sales and Operations Planning 23 /
Manufacturing Resource Planning 25 / Enterprise Resource Planning 27 / Making the Production
Plan 27 / Summary 36 / Key Terms 37 / Questions 37 / Problems 38 / Case Study 2.1:
Meridian Water Pumps 42 / Case Study 2.2: Williams 3D Printers 43
3 Master Scheduling
45
Introduction 45 / Relationship to Production Plan 46 / Developing a Master Production
Schedule 48 / Production Planning, Master Scheduling, and Sales 53 / Summary 59 /
Key Terms 59 / Questions 59 / Problems 60 / Case Study 3.1: Acme Water Pumps 66 / Case
Study 3.2: The MasterChip Electronics Company 67 / Case Study 3.3: Macarry’s Bicycle Company 69
4 Material Requirements Planning
72
Introduction 72 / Bills of Material 74 / Material Requirements Planning Process 81 / Using the
Material Requirements Plan 92 / Summary 96 / Key Terms 96 / Questions 96 / Problems 97 /
Case Study 4.1: Apix Polybob Company 108 / Case Study 4.2: Benzie Products Company 110
5 Capacity Management
112
Introduction 112 / Definition of Capacity 112 / Capacity Planning 113 / Capacity Requirements
Planning 114 / Capacity Available 116 / Capacity Required (Load) 119 / Scheduling
Orders 122 / Making the Plan 123 / Summary 125 / Key Terms 125 / Questions 126 /
Problems 127 / Case Study 5.1: Wescott Products 130
6 Production Activity Control
133
Introduction 133 / Data Requirements 136 / Order Preparation 137 / Scheduling 138 /
Load Leveling 143 / Scheduling in a Nonmanufacturing Setting 144 / Scheduling Bottlenecks 144 /
Theory of Constraints and Drum-Buffer-Rope 146 / Implementation 149 / Control 150 /
Production Reporting 155 / Product Tracking 156 / Measurement Systems 156 / Summary 156 /
Key Terms 157 / Questions 157 / Problems 158 / Case Study 6.1: Johnston Products 162 /
Case Study 6.2: Crofts Printing Company 164 / Case Study 6.3: Melrose Products 165
v
vi
Contents
7 Purchasing
168
Introduction 168 / Establishing Specifications 171 / Functional Specification Description 173 /
Selecting Suppliers 175 / Price Determination 178 / Impact of Material Requirements Planning on
Purchasing 180 / Environmentally Responsible Purchasing 182 / Expansion of Purchasing into Supply
Chain Management 183 / Some Organizational Implications of Supply Chain Management 185 /
Summary 186 / Key Terms 186 / Questions 186 / Problems 187 / Case Study 7.1: Let’s
Party! 187 / Case Study 7.2: The Connery Company 188
8 Forecasting and Demand Management
190
Introduction 190 / Demand Management 190 / Demand Forecasting 192 / Characteristics of
Demand 192 / Principles of Forecasting 194 / Collection and Preparation of Data 195 / Forecasting
Techniques 195 / Some Important Intrinsic Techniques 197 / Seasonality 200 / Tracking the
Forecast 203 / Summary 210 / Key Terms 210 / Questions 210 / Problems 211 / Case
Study 8.1: Northcutt Bikes: The Forecasting Problem 217 / Case Study 8.2: Hatcher Gear Company 219
9 Inventory Fundamentals
221
Introduction 221 / Aggregate Inventory Management 221 / Item Inventory Management 221 /
Inventory and the Flow of Material 222 / Supply and Demand Patterns 223 / Functions of
Inventories 223 / Objectives of Inventory Management 225 / Inventory Costs 227 / Financial
Statements and Inventory 229 / ABC Inventory Control 234 / Summary 237 / Key Terms 237 /
Questions 238 / Problems 239 / Case Study 9.1: Randy Smith, Inventory Control Manager 242
10 Order Quantities
245
Introduction 245 / Economic Order Quantity 246 / Variations of the EOQ Model 250 / Quantity
Discounts 251 / Order Quantities for Families of Product When Costs are Not Known 252 / Period
Order Quantity 253 / Summary 256 / Key Terms 256 / Questions 256 / Problems 257
11 Independent Demand Ordering Systems
261
Introduction 261 / Order Point System 261 / Determining Safety Stock 263 / Determining Service
Levels 269 / Different Forecast And Lead-Time Intervals 271 / Determining When The Order Point Is
Reached 271 / Periodic Review System 273 / Distribution Inventory 275 / Summary 278 /
Key Terms 278 / Questions 279 / Problems 280 / Case Study 11.1: Carl’s Computers 286
12 Physical Inventory and Warehouse Management
289
Introduction 289 / Warehousing Management 289 / Physical Control and Security 295 / Inventory
Record Accuracy 295 / Consignment Inventory and Vendor-Managed Inventory (VMI) 301 /
Technology Applications 302 / Summary 303 / Key Terms 303 / Questions 304 /
Problems 304 / Case Study 12.1: CostMart Warehouse 308
13 Physical Distribution
311
Introduction 311 / Physical Distribution 314 / Physical Distribution Interfaces 317 /
Transportation 318 / Legal Types of Carriage 320 / Transportation Cost Elements 321 /
Warehousing 326 / Packaging 331 / Material Handling 333 / Multi-Warehouse Systems 333 /
Summary 336 / Key Terms 336 / Questions 337 / Problems 338 / Case Study 13.1: Metal
Specialties, Inc. 339
Contents
14 Products and Processes
vii
341
Introduction 341 / Need for New Products 341 / Product Development Principles 342 / Product
Specification and Design 344 / Process Design 346 / Factors Influencing Process Design 347 /
Processing Equipment 349 / Process Systems 349 / Process Costing 351 / Selecting the
Process 352 / Continuous Process Improvement 354 / Summary 364 / Key Terms 365 /
Questions 365 / Problems 367 / Case Study 14.1: Cheryl Franklin, Production Manager 370
15 Lean Production
372
Introduction 372 / Lean Production 372 / Waste 374 / The Lean Production Environment 376 /
Manufacturing Planning and Control in a Lean Production Environment 383 / Comparing ERP, Kanban,
and Theory of Constraints 395 / Summary 397 / Key Terms 398 / Questions 398 /
Problems 399 / Case Study 15.1: Murphy Manufacturing 401
16 Total Quality Management
404
Introduction 404 / What Is Quality? 404 / Total Quality Management 406 / Quality Cost
Concepts 410 / Variation as a Way of Life 411 / Process Capability 413 / Process Control 417 /
Sample Inspection 420 / ISO 9000:2015 422 / ISO 26000:2010 423 / ISO 14001:2015 424 /
Benchmarking 424 / Six Sigma 425 / Quality Function Deployment 426 / The Relationship of
Lean Production, TQM, and ERP 428 / Summary 429 / Key Terms 429 / Questions 430 /
Problems 431 / Case Study 16.1: Accent Oak Furniture Company 432
Readings
Index
437
441
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PrEfaCE
Introduction to Materials Management is an introductory text written for students in
community colleges and universities. It is used in technical programs, such as industrial
engineering and manufacturing engineering; in business, operations and supply chain
management programs; and by those already in industry, whether or not they are working
in materials management.
This text has been widely adopted by colleges and universities not only in North
America but also in many other parts of the world. The APICS organization recommends
this text as a key reference for certification preparation for various CPIM examinations. In
addition, the text is used by production and inventory control societies around the world,
including South Africa, Australia, New Zealand, Germany, France, and Brazil, and by
consultants who present in-house courses to their customers.
Introduction to Materials Management covers all the basics of supply chain management, manufacturing planning and control systems, purchasing, physical distribution,
lean and quality management. The material, examples, questions, and problems lead the
student logically through the text. The writing style is simple and user-friendly—both
instructors and students who have used the book attest to this.
nEw to this Edition
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All chapters have been updated to reflect new techniques and technology
Nine additional case studies have been added
Several special topic boxes have been added relating chapter topics to nonmanufacturing settings such as service industries
End-of-chapter problems have been revised, and some new ones added throughout
the text
Expansion of purpose and impact of strategic planning, including environmental and
sustainability issues. Allows students to understand the importance of the field at a
higher level, including impacts and benefits to society as a whole
Additional information included on demand management
Additional information included on lean production concepts and Theory of Constraints.
Theory of Constraint provides an interesting and potentially effective alternative method
to think about several of the concepts in the book, and can help students compare and
contrast Theory of Constraint with non-Theory of Constraint approaches. (See Ch. 6)
A brief introduction to Project Management has been added to Ch. 6 to provide students
initial exposure to a skill today’s employers are looking for
In addition, we have retained several features from previous editions.
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Margin icons to note key concepts
Key terms listed at the end of each chapter
Example problems within the chapters
Chapter summaries
Questions and problems at the end of each chapter
Full supplements package including Instructor’s Manual, Computerized Test Bank,
PowerPoint, and Image Bank available for download
ix
x
Preface
aPProaCh and organization
Materials management means different things to different people. In this textbook, materials management includes all activities in the flow of materials from the supplier to the
consumer. Such activities include physical supply, operations planning and control, and
physical distribution. Other terms sometimes used in this area are business logistics and
supply chain management. Often, the emphasis in business logistics is on transportation
and distribution systems with little concern for what occurs in the factory. Whereas some
chapters in this text are devoted to transportation and distribution, emphasis is placed on
operations planning and control.
Distribution and operations are managed by planning and controlling the flow of
materials through them and by using the system’s resources to achieve a desired customer
service level. These activities are the responsibility of materials management and affect
every department in a manufacturing business. If the materials management system is
not well designed and managed, the distribution and manufacturing system will be less
effective and more costly. Anyone working in manufacturing or distribution should have
a good basic understanding of the factors influencing materials flow. This text aims to
provide that understanding and also includes chapters on quality management and lean
production.
APICS defines the body of knowledge, concepts, and vocabulary used in production and inventory control. Establishing standard knowledge, concepts, and vocabulary is
essential both for developing an understanding of production and inventory control and
for making clear communication possible. Where applicable, the definitions and concepts
in this text subscribe to APICS vocabulary and concepts.
The first six chapters of Introduction to Materials Management cover the basics of
production planning and control. Chapter 7 discusses important factors in purchasing
and supply chain; Chapter 8 discusses forecasting. Chapters 9, 10, and 11 look at the
fundamentals of inventory management. Chapter 12 discusses physical inventory and
warehouse management, and Chapter 13 examines the elements of distribution systems,
including transportation, packaging, and material handling. Chapter 14 covers factors
influencing product and process design. Chapter 15 looks at the philosophy and environment of lean production and explains how operations planning and control systems relate
to lean production. Chapter 16 examines the elements of total quality management and six
sigma quality approaches.
onlinE instruCtor rEsourCEs
To access supplementary materials online, instructors need to request an instructor access
code. Go to www.pearsonhighered.com, click the Instructor Resource Center link, and
then click Register Today for an instructor access code. Within 48 hours after registering you will receive a confirming e-mail including an instructor access code. Once you
have received your code, go to the site and log on for full instructions on downloading the
materials you wish to use.
List of Supplements
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Instructor’s Manual
Computerized Test Bank
PowerPoint
Image Bank
aCknowlEdgmEnts
The period of time since the seventh edition of this book was published included the very
unfortunate passing of two of the authors of the seventh edition—Tony Arnold and Lloyd
Clive. Tony Arnold was responsible for the original vision and creation of the book many
Preface
xi
years ago, and Lloyd Clive brought significant additional insights and knowledge in the
creation of the last two revisions. Both of these gentlemen were well known and highly
respected both by students and colleagues, and will be greatly missed.
The addition of Ann Gatewood as a new coauthor brings her extensive experience,
knowledge, and insight to this eighth edition. However, this eighth edition continues to
reflect the original vision of providing a clear and understandable introductory look at the
field of Materials Management.
Help and encouragement have come from a number of valued sources, among
them friends, colleagues, and students. We thank the many readers of the book who
have provided comments and suggestions. We especially wish to thank members of the
various APICS CPIM Committees who have provided specific guidance for the revision.
Specifically, we would like to thank Andrea Prud’homme (The Ohio State University),
Jim Caruso (Covidien), Frank Montabon (Iowa State University), and Mark Hardison
(SIGA Technologies) for their significant insights and suggestions. In addition, we
received several worthwhile suggestions from John Kanet (The University of Dayton) and
Keith Launchbury (Keith Launchbury and Associates). Other academic reviewers include
Vahid H Khiabani (Minnesota State University—Moorhead), Michael Gallaway (North
Lake College), John Kros (East Carolina University), and Sunderesh Heragu (Oklahoma
State University—Stillwater). Steve Chapman would also like to thank his wife Jeannine
for her continued support and encouragement during the revision process.
Overall, this book is dedicated to those who have taught us the most—our colleagues
and our students.
Stephen N. Chapman, Ph.D., CFPIM, Associate Professor Emeritus
Department of Business Management, Poole College of Management
North Carolina State University
Raleigh, North Carolina
Ann K. Gatewood, CFPIM, CIRM, CSCP
President, Gatewood Associates, LLC
Mooresville, North Carolina
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Chapter
One
intrOductiOn tO materials
management
intrOductiOn
The wealth of a country is measured by its gross national product—the output of goods
and services produced by the nation in a given time. Goods are physical objects, something one can touch, feel, or see. Services are the performance of some useful function
such as banking, medical care, restaurants, clothing stores, or social services.
But what is the source of wealth? Wealth is measured by the amount of goods
and services produced, but where does it come from? Although rich natural resources
may exist in an economy, such as mineral deposits, farmland, and forests, these are
only potential sources of wealth. A production function is needed to transform these
resources into useful goods. The transformation process begins with extracting minerals
from the earth, farming, lumbering, or fishing, and then using these resources to manufacture useful products.
There are many stages between the extraction of resource material and the final consumer product. At each stage in the development of the final product, value is added, thus
creating more wealth. If ore is extracted from the earth and sold, wealth is gained from the
efforts, but those who continue to transform the raw material will gain more and usually
far greater wealth. Japan is a prime example of this. It has very few natural resources and
imports most of the raw materials it needs. However, the Japanese have developed one of
the wealthiest economies in the world by transforming the raw materials they purchase
and adding value to them through manufacturing.
Manufacturing companies are in the business of converting raw materials to a form
that is of far more value and use to the consumer than the original raw materials. Logs are
converted into tables and chairs, iron ore into steel, and steel into cars and refrigerators.
This conversion process, called manufacturing or production, makes a society wealthier
and creates a better standard of living.
To get the most value out of resources, production processes must be so designed
that they make products most efficiently. Once the processes exist, operations are managed so they produce goods most economically. Managing the operation means planning
for and controlling the resources used in the process: labor, capital, and material. All
are important, but the major way in which management plans and controls operations is
through the flow of materials. The flow of materials in turn controls the performance of
the process. If the right materials in the right quantities are not available at the right time,
the process cannot produce what it should. Labor and machinery will be poorly utilized.
The profitability, and even the existence, of the company will be threatened.
Operating envirOnment
Operations management works in a complex environment affected by many factors.
Among the most important are government regulation, the economy, competition, customer expectations, and quality.
Government. Regulation of business by the various levels of government is extensive. Regulation applies to such areas as the environment, safety, product liability,
and taxation. Government, or the lack of it, affects the way business is conducted.
1
2
Chapter one
Economy. General economic conditions influence the demand for a company’s
products or services and the availability of inputs. During economic recession, the
demand for some products may decrease while demand for others may increase.
Materials and labor shortages or surpluses influence the decisions management
makes. Shifts in the age of the population, needs of ethnic groups, low population
growth, increased free trade between countries, and increased global competition
all contribute to changes in the marketplace.
Competition. Competition is more severe today than ever before.
■■ Manufacturing companies face competition from throughout the world. They
find foreign competitors selling in their markets even though they themselves
may not be selling in foreign markets.
■■ Transportation and the movement of materials are relatively more efficient and
less costly than they used to be.
■■ Worldwide communications are fast, effective, and cheap. Information and data
can be moved almost instantly around the globe. The internet allows buyers to
search out new sources of supply from anywhere in the world as easily as they
can from local sources.
Customers. Both consumers and industrial customers have become much more
demanding, and suppliers have responded by improving the range of characteristics
they offer. Some of the characteristics and selection customers expect in the products and services they buy are:
■■ A fair price.
■■ Higher (exact) quality products and services.
■■ Delivery lead time.
■■ Better presale and after-sale service.
■■ Product and volume flexibility.
Quality. Since competition is international and aggressive, successful companies
provide quality that not only meets customers’ high expectations but also exceeds
them.
Order Qualifiers and Order Winners
Generally, a supplier must meet set minimum requirements to be considered a viable
competitor in the marketplace. Customer requirements may be based on price, quality,
delivery, and so forth and are called order qualifiers. For example, the price for a certain
type of product must fall within a range for the supplier to be considered by potential customers. But being considered does not mean winning the order. To win orders, a supplier
must have characteristics that encourage customers to choose its products and services
over competitors’. Those competitive characteristics, or combination of characteristics,
that persuade a company’s customers to choose its products or services are called order
winners. They provide a competitive advantage for the firm. Order winners change over
time and may well be different for different markets. For example, fast delivery may be
vital in one market but not in another. Characteristics that are order winners today probably will not remain so, because competition will try to copy winning characteristics, and
the needs of customers will change.
It is very important that a firm understands the order winners and order qualifiers
for each of its products or services and in each of its markets because they should drive
the manufacturing and corporate strategy. Since it is virtually impossible to be the best in
every dimension of competition, firms should in general strive to provide at least a minimal level of acceptance for each of the order qualifiers but should try to be the best in the
market for the order winner(s).
One also should recognize that the order winners and qualifiers for any product/
market combination are not static. Not only will customers change perspectives as competitors jockey for position, but the order winners and qualifiers will also often change
based on the concepts of the product life cycle. Most products go through a life cycle,
Introduction to Materials Management
3
including introduction, growth, maturity, and decline. For example, in the introduction
phase, design and availability are often much more important than price. Quality and
delivery tend to have increased importance during growth, while price and delivery are
often the order winners for mature products. This life cycle approach is complicated in
that the duration of the life cycle will be very different for different products. Although
some products have life cycles many years long, the life cycle of other products (certain
toys or electronics, for example) can be measured in months or even weeks.
Manufacturing Strategy
A highly market-oriented company will focus on meeting or exceeding customer expectations and on order winners. In such a company, all functions must contribute toward a
winning strategy. Thus, operations must have a strategy that allows it to supply the needs
of the marketplace and provide fast on-time delivery.
Delivery lead time From the supplier’s perspective, delivery lead time is the time
from receipt of an order to the delivery of the product. From the customer’s perspective, it
may also include time for order preparation and transmittal. Most customers want delivery
lead time to be as short as possible, and manufacturing must determine a process strategy
to achieve this. There are five basic process strategy choices: engineer-to-order, make-toorder, configure-to-order, assemble-to-order, and make-to-stock. Customer involvement
in the product design, delivery lead time, and inventory state are influenced by each
strategy. Based on the type of products a company makes, and their customer base, a company may determine that more than one process strategy is required. Figure 1.1 shows the
effect of each process strategy on lead time.
Engineer-to-order means that the customer’s specifications require unique engineering design or significant customization. Usually the customer is highly involved in the
product design. Inventory will not normally be purchased until needed by manufacturing.
Delivery lead time is long because it includes not only purchase lead time but also design
lead time.
Make-to-order means that the manufacturer does not start to make the product until
a customer’s order is received. The final product is usually made from standard items but
may include custom-designed components as well. Delivery lead time is reduced because
there is little design time required and inventory is held as raw material.
Delivery Lead Time
DESIGN
PURCHASE
MANUFACTURE
ASSEMBLE
SHIP
ENGINEER-TOORDER
SHIP
MAKE-TOORDER
SHIP
CONFIGURETO-ORDER
Delivery Lead Time
INVENTORY
MANUFACTURE
ASSEMBLE
Delivery Lead Time
INVENTORY
MANUFACTURE
ASSEMBLE
Delivery Lead Time
MANUFACTURE
ASSEMBLE
SHIP
ASSEMBLETO-ORDER
INVENTORY
SHIP
MAKE-TOSTOCK
INVENTORY
Delivery Lead Time
MANUFACTURE
ASSEMBLE
FIgure 1.1 Manufacturing strategy and lead time.
4
Chapter one
Configure-to-order means that the customer is allowed to configure a product based
on various features and options. Each customer, and order, may be an entirely unique
configuration that has never been done before, and the configuration often occurs at
the beginning of the process. Delivery lead time is reduced because there is no design
time required and the different features and options may already be available. Customer
involvement includes selecting the features and options desired.
Assemble-to-order means that the product is made from standard components or
options that the manufacturer can inventory and assemble according to a customer order.
This is usually done at a later stage in the process than configure-to-order. Delivery lead
time is reduced further because there is no design time needed and inventory is held ready
for assembly. Customer involvement in the design of the product is limited to selecting the
assembly options needed.
Make-to-stock means that the supplier manufactures the goods and sells from a finished goods inventory. Delivery lead time is shortest as manufacturing and assembly have
already been completed. The customer has little direct involvement in the product design.
Postponement is another application of assemble-to-order, described in APICS
Dictionary, 14th edition as “a product design strategy that shifts product differentiation
closer to the consumer by postponing identity change to the last possible supply chain
location.” This strategy reduces the number of different items in the supply chain, lowering the amount of in-process inventory.
An example of postponement would be computer printers for a global market that
use universal power supplies that can be switched to different voltages. Upon receipt of a
customer’s order, they are packaged with the appropriate cords, instructions, and labeling.
This avoids filling an entire supply chain with expensive printers destined for many different countries. Some basic postponement can be done in a distribution center and often
by third party logistics (3PL) providers. Foreign suppliers of appliances, such as vacuum
cleaners destined for multiple customers, postpone the packaging of their products, applying customer-specific labels, bar codes, boxes, instructions, and so forth until after receipt
of the customer order.
the supply chain cOncept
There are three phases to the flow of materials. Raw materials flow into a manufacturing company from a physical supply system, they are processed by manufacturing, and
finally, finished goods are distributed to end consumers through a physical distribution
system. Figure 1.2 shows this system graphically. Although this figure shows only one
supplier and one customer, usually the supply chain consists of several companies linked
in a supply–demand relationship. For example, the customer of one supplier buys a product, adds value to it, and supplies it to yet another customer. Similarly, one customer may
S
U
P
P
L
I
E
R
MANUFACTURER
Physical
Supply
Manufacturing,
Planning, and
Control
DISTRIBUTION
SYSTEM
Physical Distribution
DOMINANT FLOW OF PRODUCTS AND SERVICES
DOMINANT FLOW OF DEMAND AND DESIGN INFORMATION
FIgure 1.2 Supply–production–distribution system.
C
U
S
T
O
M
E
R
Introduction to Materials Management
5
have several suppliers and may in turn supply several customers. As long as there is a
chain of supplier–customer relationships, they are all members of the same supply chain.
There are a number of important factors in supply chains:
The supply chain includes all activities and processes to supply a product or service to
a final customer.
■■ Any number of companies can be linked in the supply chain.
■■ A customer can be a supplier to another customer, so the total chain can have a number of supplier–customer relationships.
■■ Although the distribution system can be direct from supplier to customer, depending
on the products and markets, it can contain a number of intermediaries (distributors)
such as wholesalers, warehouses, and retailers.
■■ Product or services usually flow from supplier to customer; design, demand information, and cash usually flow from customer to supplier.
■■
Although these systems vary from industry to industry and company to company, the
basic elements are the same: supply, production, and distribution. The relative importance
of each depends on the costs of the three elements.
Supply Chain Concepts
In recent years there has been a great deal of attention given to the concept of supply
chain management (SCM). It is important to understand fundamental concepts of supply
chain management and its impact on materials management.
Historical perspective In the past, many company managers placed most of their
attention on the issues that were internal to their companies. Of course, they were aware of
the impact of suppliers, customers, and distributors, but those entities were often viewed
as business entities only. Specialists in purchasing, sales, and logistics were assigned
to deal with those outside entities, often through formal legal contracts that were negotiated regularly and represented short term agreements. For example, suppliers were
often viewed as business adversaries. A key responsibility of a purchasing agent was to
negotiate the best financial and delivery conditions from a supplier, whose job was to
maximize company profit.
The first major change in that perspective for most companies can be traced to the
explosive growth in just-in-time (JIT) concepts, originally developed by Toyota and other
Japanese companies in the 1970s. Supplier partnerships were felt to be a major aspect of
successful JIT. With that concept, suppliers were viewed as partners as opposed to adversaries, meaning the supplier and the customer had mutually linked destinies, and each was
linked to the success of the other. Great emphasis was put on trust between the partners,
and many of the formal boundary mechanisms, such as the receiving/inspection activity of
incoming parts, were changed or eliminated altogether. As the partnership concept grew,
there were many other changes in the relationship, including:
Mutual analysis for cost reduction. Both parties examined the process used to
transmit information and deliver parts, with the idea that cost reductions would be
shared between the two parties.
■■ Mutual product design. In the past, the customer often submitted complete designs
to the supplier, who was obligated to produce according to design. With partnering,
both companies worked together. Often the supplier would know more about how
to make a specific product, whereas the customer would know more about the application for which the design was intended. Together, they could produce a superior
design compared to what either could do alone.
■■ Enhanced information flow. JIT incorporated the concept of greatly reduced inventory in the process and the need for rapid delivery according to need; therefore, the
speed of accurate information flow became critical. Formal paper-based systems gave
way to electronic data interchange (EDI) and more informal communication methods
between individuals at the supplier and customer.
■■
6
Chapter one
The growth of the supply chain concept As the world continues to change, additional modifications are being added to the trend:
There has been explosive growth in computer capability and associated software
applications. Highly effective and integrated systems such as enterprise resource
planning (ERP) and the ability to link companies electronically (through the internet,
for example) have allowed companies to share large amounts of information quickly
and easily. The ability to have information rapidly has become a competitive necessity for many companies.
■■ There has been a large growth in global competition. Very few companies can still
say they have only local competition, and many of the global competitors are forcing
existing companies to find new ways to be successful in the marketplace.
■■ There has been a growth in technological capabilities for products and processes.
Product life cycles for many products are shrinking rapidly, forcing companies to not
only become more flexible in design but also to communicate changes and needs to
suppliers and distributors.
■■ The changes prompted by JIT in the 1980s have continued to mature and become more
accurately defined as lean production. Now many companies have new approaches to
inter-organizational relationships as a normal form of business.
■■ Partially in response to the preceding conditions, more and more companies are subcontracting more of their work to suppliers, keeping only their most important core
competencies as internal activities.
■■
What is the current supply chain philosophy? Companies adopting the supply chain
concept now view the entire set of activities from raw material production to final customer
purchase, to final disposal as a linked chain of activities. To yield optimal performance for
customer service and cost, it is felt that the supply chain of activities should be managed as an
extension of the partnership. This implies many issues, but three critical ones are as follows:
1. Flow of materials.
2. Flow and sharing of information.
3. Flow of funds.
In addition, a new trend is emerging to manage the recovery, recycling, and reuse of
material, known as reverse logistics.
The primary supply chain management approach is a conceptual one. All portions of
the material production, from raw materials to final customer, are considered to be in a
linked chain. The most efficient and effective way to manage the activities along the chain
is to view each separate organization in the chain as an extension of one’s own organization. There can be many organizations in a supply chain. Take as an example the chain of
organizations that represents the flow from raw silicon used to make computer chips to the
delivery and disposal of the computer itself in Figure 1.3.
What is illustrated here is but one chain of a set of different component chains that
represent a network of suppliers and distributors for a product.
Most companies work with a network of supply chains, obtaining a variety of materials from multiple suppliers and sending products to multiple customers. Even a grocery
SILICON
PRODUCTION
INTEGRATED
CIRCUIT (CHIP)
PRODUCTION
DISTRIBUTOR
RETAILER
FIgure 1.3 Supply chain organizations.
PRINTED CIRCUIT
BOARD PRODUCTION
CUSTOMER
COMPUTER
PRODUCTION
DISPOSAL
Introduction to Materials Management
7
store has to deal with suppliers of dry goods, magazines, frozen and fresh products, and
small suppliers of local produce or specialty goods.
The many independent businesses that make up a supply chain have individual profit
motives and do not naturally cooperate to gain savings. This requires someone to take the initiative. Any member of the supply chain can work with other members to show the benefits of
sharing information on forecasts, sales information, or schedules. Orchestrator or channel
master are two emerging terms that describe the individual or company that takes the initiative to integrate both the upstream and downstream supply chain, getting members to work
cooperatively to lower total costs and achieve greater efficiency. This is often the nucleus firm
within the supply chain. The result is a network of companies that openly share information.
To manage a supply chain, one must not only understand the network of suppliers and
customers along the chain but also try to efficiently plan material and information flows
along each chain to maximize cost efficiency, effectiveness, delivery, and flexibility. This
clearly implies not only taking a different conceptual approach to suppliers and customers but also a highly integrated information system and a different set of performance
measures. Overall, the key to managing such a concept is with rapid flows of accurate
information and increased organizational flexibility.
Supply Chain Metrics
A metric is a verifiable measure stated in either quantitative or qualitative terms defined
with respect to a reference point. Without metrics, no firm can expect to function effectively or efficiently on a daily basis. Metrics give us
1. Control by superiors.
2. Reporting of data to superiors and external groups.
3. Communication.
4. Learning.
5. Improvement.
Building the right metrics is vital to a company, as metrics communicate expectations, identify problems, direct a course of action, and motivate people. Problems must be
anticipated and corrective action taken before they become severe and costly. Companies
cannot risk waiting to react until the order cycle is completed and feedback from customers is received.
Today, production control works in a demanding environment shaped by six major
challenges:
1. Customers that are rarely satisfied.
2. A supply chain that is large and must be managed.
3. A product life cycle that is getting shorter and shorter.
4. A vast amount of data.
5. An emphasis on profit margins that are more squeezed.
6. An increasing number of alternatives.
A firm typically has a corporate strategy that states how it will treat its customers and
what services it will supply. This identifies how a firm will compete in the marketplace. It
is the customer who assesses the firm’s offering by its decision to buy or not to buy. How
metrics link strategy to operations is shown in Figure 1.4. Focus describes the particular
Strategy
Customer
Strategic
FIgure 1.4 Metrics context.
Metrics
Operational
Focus
Standard
8
Chapter one
activity that is to be measured. Standards are the yardstick that is the basis of comparison
on which performance is judged.
There is a difference between performance measurements and performance standards.
A performance measure must be both quantified and objective and contain at least two
parameters. For example, the number of orders per day consists of both a quantity and
a time measurement.
Transforming company policies into objectives and specific goals creates performance
standards. Each goal should have target values. An example of this would be to improve
order fill rate to 98% measured by number of lines. Performance standards set the goal,
while performance measures reveal how close to the goal the organization reached.
Many companies do not realize the potential benefits of performance measurement,
nor do they know how to measure performance, and often try to use them without performance standards. This might occur when the concept of performance measurement and
standards is new. Only when standards are put into use can management begin to monitor
the company. The old saying “What you do not measure, you cannot control” is as valid
today as it was when first stated.
The necessary steps in implementing such a program are as follows:
1. Establish company goals and objectives.
2. Define performance.
3. State the measurement to be used.
4. Set performance standards.
5. Educate the participant.
6. Make sure the program is consistently applied.
Although financial performance has traditionally been the measure of success in most
companies, today the focus is on continuous improvement and, with this, an increase
in standards. Emphasis should not be placed on a “one-shot” improvement but on such
things as the rate of improvement in quality, cost, reliability, innovation, effectiveness,
and productivity.
Conflicts in Traditional Systems
In the past, supply, production, and distribution systems were organized into separate
functions that reported to different departments of a company. Often, policies and practices of the different departments maximized departmental objectives without considering
the effect they would have on other parts of the system. Because the three systems are
interrelated, conflicts often occurred. Although each system made decisions that were best
for itself, overall company objectives suffered. For example, the transportation department
would ship in the largest quantities possible so it could minimize per-unit shipping costs.
However, this increased inventory and resulted in higher inventory-carrying costs.
To get the most profit, a company must have at least four main objectives:
1. Provide best customer service.
2. Provide lowest production costs.
3. Provide lowest inventory investment.
4. Provide lowest distribution costs.
These objectives create conflict among the marketing, production, and finance departments because each has different responsibilities in these areas.
Marketing’s objective is to maintain and increase revenue; therefore, it must provide
the best customer service possible. There are several ways of doing this:
Maintain high inventories so goods are always available for the customer.
Interrupt production runs so that a non-inventoried item can be manufactured quickly.
■■ Create an extensive, and consequently costly, distribution system so goods can be
shipped to the customer rapidly.
■■
■■
Introduction to Materials Management
FUNCTION
OBJECTIVE
IMPLICATION
Marketing
• High Revenues
• High Product
Availability
High
Production
• Low Production
Cost
• High-Level
Production
• Long Production
Runs
Many Disruptions
to
Few
Production
Finance
• Low Investment
and Cost
• Fewer Fixed
Costs
• Low Inventories
Low
High
Low
9
Customer Service
Inventories
FIgure 1.5 Conflicting objectives.
Finance must keep investment and costs low. This can be done in the following ways:
Reduce inventory so inventory investment is at a minimum.
Decrease the number of plants and warehouses.
■■ Produce large quantities using long production runs.
■■ Manufacture only to customer order.
■■
■■
Production must keep its operating costs as low as possible. This can be done in the
following ways:
Make long production runs of relatively few products. Fewer changeovers will be
needed and specialized equipment can be used, thus reducing the cost of making the
product.
■■ Maintain high inventories of raw materials and work-in-process so production is not
disrupted by shortages.
■■
These conflicts among marketing, finance, and production center on customer service,
disruption of production flow, and inventory levels. Figure 1.5 shows this relationship.
Today, the concepts of lean production stress the need to supply customers with what
they want, when they want it, and to keep inventories at a minimum. These objectives put
further stress on the relationship among production, marketing, and finance. Chapter 15
will discuss the concepts of lean production and how it influences materials management.
One important way to resolve these conflicting objectives is to provide close coordination of the supply, production, and distribution functions. The problem is to balance
conflicting objectives to minimize the total of all the costs involved and maximize
customer service consistent with the goals of the organization. This requires some type of
integrated materials management or logistics organization that is responsible for supply,
production, and distribution. Rather than having the planning and control of these functions spread among marketing, production, and distribution, they should occur in a single
area of responsibility.
What is materials management?
The concept of having one department responsible for the flow of materials, from supplier
through production to consumer, thereby minimizing total costs and providing a better
level of customer service, is known as materials management. Other names include distribution planning and control, supply chain management, and logistics management, but
the one used in this text is materials management. As will be discussed in Chapter 15, lean
production not only requires efficient individual operations but also requires all operations
to work together. A materials management department can improve this coordination by
having overall responsibility for material.
10
Chapter one
Materials management is a coordinating function responsible for planning and controlling materials flow. Its objectives are as follows:
■■
■■
Maximize the use of the firm’s resources.
Provide the required level of customer service.
Materials management can do much to improve a company’s profit. An income
(profit and loss) statement for a manufacturing company might look something like the
following:
Revenue (sales)
Cost of Goods Sold
Direct Material
Direct Labor
Factory Overhead
Total Cost of
Goods Sold
Gross Profit
Dollars
$1,000,000
$ 500,000
$ 200,000
$ 200,000
Percent
of Sales
100
50
20
20
$ 900,000
$100,000
90
10
Direct labor and direct material are costs that increase or decrease with the quantity
sold. Overhead (all other costs) does not vary directly with sales. For simplicity, this
section assumes overhead is constant, even though it is initially expressed as a percentage of sales.
If, through a well-organized materials management department, direct materials can
be reduced by 12%, the improvement in profit would be
Revenue (sales)
Cost of Goods Sold
Direct Material
Direct Labor
Overhead
Total Cost of
Goods Sold
Gross Profit
Dollars
$1,000,000
$ 440,000
$ 200,000
$ 200,000
Percent
of Sales
100
44
20
20
$ 840,000
$160,000
84
16
Profit has been increased by 60%. In other words, managing inventory effectively
goes right to the bottom line of a company’s profit. To get the same increase in profit
($60,000) by increasing revenue, sales would have to increase to $1.2 million.
Revenue (sales)
Cost of Goods Sold
Direct Material
Direct Labor
Overhead
Total Cost of
Goods Sold
Gross Profit
Dollars
$1,200,000
$ 600,000
$ 240,000
$ 200,000
Percent
of Sales
100
50
20
17
$1,040,000
$160,000
87
13
Introduction to Materials Management
11
example Problem
a. If the cost of direct material is 60%, direct labor is 10%, and overhead is 25% of
sales, what will be the improvement in profit if cost of direct material is reduced to
55%?
b. How much will sales have to increase to give the same increase in profit? (Remember,
overhead cost is constant.)
Answer
Before
Improvement
a.
Revenue (sales)
After
Improvement
100%
100%
Cost of Goods Sold
b.
Direct Material
60%
55%
Direct Labor
10%
10%
Overhead
25%
25%
Total Cost of Goods Sold
95%
90%
Gross Profit
5%
10%
Profit = sales - (direct material + direct labor + 0.25)
= sales - (0.6 sales + 0.1 sales + 0.25)
= sales - 0.7 sales - 0.25
0.1 = 0.3 sales - 0.25
0.3 Sales = 0.35
Sales =
0.35
= 1.17
0.3
Sales must increase 17% to give the same increase in profit.
Work-in-Process
Inventory not only accounts for the raw materials and purchased components, but is also
made up of the product as it is processed into finished goods. This type of inventory is
called work-in-process (WIP). WIP is a major investment for many companies, and
reducing the amount of time that inventory spends in production is a good way to reduce
the costs associated with this investment. Labor, materials, and overhead are applied to
goods continuously through-out production, which increases the value of WIP. Further
discussion on WIP and reducing it is covered in Chapters 9 and 15.
example Problem
On average, a company has a 12-week production lead time and an annual cost of
goods sold of $36 million. Assuming the company works 50 weeks per year:
a. What is the dollar value of the WIP?
b. If the lead time could be reduced to 5 weeks, and the annual cost of carrying
inventory was 20% of the inventory value, what would be the annual savings?
Answer
Weekly cost of goods sold = $ 36,000,000 per year/50 weeks per year
= $ 720,000/week
WIP value at 12 weeks LT = 12 weeks * $ 720,000/week = $ 8,640,000
WIP value at 5 weeks LT = 5 weeks * $ 720,000/week = $ 3,600,000
Reduction in WIP = $ 8,640,000 - 3,600,000 = 5,040,000
Annual Savings = $ 5,040,000 * 20 % = $ 1,008,000
12
Chapter one
Reducing cost contributes directly to profit. Increasing sales increases direct costs of
labor and materials so profit does not increase in direct proportion. Materials management
can reduce costs by being sure that the right materials are in the right place at the right
time and the resources of the company are properly used.
There are several ways of classifying this flow of material. A very useful classification, and the one used in this text, is manufacturing planning and control and physical
supply/distribution.
Manufacturing Planning and Control
Manufacturing planning and control are responsible for the planning and control of the
flow of materials through the manufacturing process. The primary activities carried out
are as follows:
1. Production planning. Production must be able to meet the demand of the marketplace. Finding the most productive way of doing so is the responsibility of production
planning. It must establish correct priorities (what is needed and when) and make
certain that capacity is available to meet those priorities. It involves:
a. Forecasting.
b. Master planning.
c. Material requirements planning.
d. Capacity planning.
2. Implementation and control. These functions are responsible for putting into
action and executing the plans made by production planning. These responsibilities
are accomplished through production activity control (often called shop floor control)
and purchasing.
3. Inventory management. Inventories are materials and supplies carried on hand
either for sale or to provide material or supplies to the production process. They are
part of the planning process and provide a buffer against the differences in demand
rates and production rates.
Production planning, implementation, control, and inventory management work handin-hand. Inventories in manufacturing are used to support production or are the result of
production.
Inputs to the manufacturing planning and control system There are five basic
inputs to the manufacturing planning and control system:
1. Product description. The product description shows how the product will appear
at some stage of production. Engineering drawings and specifications are methods of
describing the product. Another method, and the most important for manufacturing
planning and control, is the bill of material. As used in materials management, this
document does two things:
■■ Describes the components used to make the product.
■■ Describes the subassemblies at various stages of manufacture.
2. Process specifications. Process specifications describe the steps necessary to make
the end product. They are a step-by-step set of instructions describing how the product is made. This information is usually recorded on a route sheet or in a routing.
These are documents or computer files that give information such as the following on
the manufacture of a product:
Operations required to make the product.
Sequence of operations.
■■ Equipment and accessories required.
■■ Standard time required to perform each operation.
■■
■■
Introduction to Materials Management
13
3. Time. The time needed to perform operations is usually expressed in standard time,
which is the time taken by an average operator, working at a normal pace, to perform
a task. It is needed to schedule work through the plant, load the plant, make delivery
promises, and cost the product. Standard times for operations are usually obtained
from the routing information.
4. Available facilities. Manufacturing planning and control must know what plant,
equipment, and labor will be available to process work. This information is usually
found in the work center information.
5. Quantities required. This information will come from forecasts, customer orders,
orders to replace finished goods inventory, and the material requirements plan.
Physical Supply/Distribution
Physical supply/distribution includes all the activities involved in moving goods, from the
supplier to the beginning of the production process, and from the end of the production
process to the consumer.
The activities involved are as follows:
Transportation.
Distribution inventory.
■■ Warehousing.
■■ Packaging.
■■ Material handling.
■■ Order entry.
■■
■■
Materials management is a balancing act. The objective is to be able to deliver what
customers want, when and where they want it, and to do so at minimum cost. To achieve
this objective, materials management must make tradeoffs between the level of customer
service and the cost of providing that service. As a rule, costs rise as the service level
increases, and materials management must find that combination of inputs to maximize
service and minimize cost. For example, customer service can be improved by establishing warehouses in major markets. However, that causes extra cost in operating the
warehouse and in the extra inventory carried. To some extent, these costs will be offset by
potential savings in transportation costs if lower cost transportation can be used.
By grouping all those activities involved in the movement and storage of goods into one
department, the firm has a better opportunity to provide maximum service at minimum cost
and to increase profit. The overall concern of materials management is the balance between
priority and capacity. The marketplace sets demand and materials management must plan
the firm’s priorities (what goods to make and when) to meet that demand. Capacity is the
ability of the system to produce or deliver goods. Materials management is responsible for
planning and controlling priority and capacity to meet customer demand at minimum cost.
summary
Manufacturing creates wealth by adding value to goods. To improve productivity and
wealth, a company must first design efficient and effective systems for manufacturing. It
must then manage these systems to make the best use of labor, capital, and material. One
of the most effective ways of doing this is through the planning and control of the flow of
materials into, through, and out of manufacturing. There are three elements to a material
flow system: supply, manufacturing planning and control, and physical distribution. They
are connected, and what happens in one system affects the others.
Traditionally, there are conflicts in the objectives of a company and in the objectives
of marketing, finance, and production. The role of materials management is to balance
these conflicting objectives by coordinating the flow of materials so customer service is
maintained and the resources of the company are properly used.
This text will examine some of the theory and practice considered to be part of the
body of knowledge of materials and supply chain management.
14
Chapter one
Key terms
Assemble-to-order 4
Available facilities 13
Bill of material 12
Channel master 7
Configure-to-order 4
Engineer-to-order 3
Enterprise resource planning (ERP) 6
Implementation and control 12
Inventory management 12
Make-to-order 3
Make-to-stock 4
Materials management 9
Metric 7
Orchestrator 7
Order qualifiers 2
Order winners 2
Performance measure 8
Performance standards 8
Postponement 4
Process specifications 12
Product description 12
Production planning 12
Quantities required 13
Reverse logistics 6
Routing 12
Standard time 13
Supply chain management 5
Work-in-process (WIP) 11
QuestiOns
1. What is wealth, and how is it created?
2. What is value added, and how is it achieved?
3. Name and describe four major factors affecting operations management.
4. What are an order qualifier and an order winner?
5. Describe the five primary manufacturing strategies. How does each affect delivery lead time?
6. What is a supply chain? Describe five important factors in supply chains.
7. What must manufacturing management do to manage a process or operation? What is the major
way in which management plans and controls?
8. Name and describe the three main divisions of supply, production, and distribution systems.
9. What are the four objectives of a firm wishing to maximize profit?
10. What is the objective of marketing? What three ways will help it achieve this objective?
11. What are the objectives of finance? How can these objectives be met?
12. What are the objectives of production? How can these objectives be met?
13. Describe how the objectives of marketing, production, and finance are in conflict over customer service, disruption to production, and inventories.
14. What is the purpose of materials management?
15. Name and describe the three primary activities of manufacturing planning and control.
16. Name and describe the inputs to a manufacturing planning and control system.
17. What are the six activities involved in the physical supply/distribution system?
18. Why can materials management be considered a balancing act?
19. What are metrics? What are their uses?
20. A computer carrying case and a backpack are familiar items to a student of manufacturing
planning and control. Discuss the manufacturing planning and control activities involved in
producing a variety of these products. What information from other departments is necessary
for manufacturing planning and control to perform its function?
21. Describe at least three supply chains that provide products to your school book store. Do they
use cooperative supply chain methods to help reduce their costs?
22. From the bookstore example in question 21, describe how one of the supply chains would use a
supply chain “channel master.”
Introduction to Materials Management
15
23. Which manufacturing strategies are used in a fast-food business? How does this affect the lead
time from the customers’ point of view?
24. Give an example of a postponement activity.
prOblems
1.1 If the cost of manufacturing (direct material and direct labor) is 60% of sales and profit
is 10% of sales, what would be the improvement in profit if, through better planning and
control, the cost of manufacturing was reduced from 60% of sales to 50% of sales?
Answer. Profits would improve by 100%.
1.2 In problem 1.1, how much would sales have to increase to provide the same increase
in profits?
Answer. Sales would have to increase 25%.
1.3 On the average, a firm has an 8-week lead time for work-in-process, and annual cost
of goods sold is $12 million. Assuming that the company works 50 weeks a year:
a. What is the dollar value of the work-in-process?
b. If the lead time could be reduced to 6 weeks, what would be the reduction in WIP?
Answer. a. $1,920,000
c. $480,000
1.4 On the average, a company has a work-in-process lead time of 10 weeks and annual
cost of goods sold of $30 million. Assuming that the company works 50 weeks a year:
a. What is the dollar value of the work-in-process?
b. If the work-in-process could be reduced to 5 weeks and the annual cost of carrying
inventory was 20% of the WIP inventory value, what would be the annual savings?
1.5 Amalgamated Fenderdenter’s sales are $10 million. The company spends $3.5 million
for purchase of direct materials and $2.5 million for direct labor; overhead is $3.5 million
and profit is $500,000. Direct labor and direct material vary directly with sales, but
overhead does not. The company wants to double its profit.
a. By how much should the firm increase annual sales?
b. By how much should the firm decrease material costs?
c. By how much should the firm decrease labor cost?
case study 1.1
Fran’s Flowers
After Fran graduated with an undergraduate art degree in 2008, she decided to combine
her knowledge and love of art with a second love—plants and flowers—toward developing a business. Her intent was to focus on a specialty niche in the flower shop business.
She decided to concentrate her efforts on make-to-order special flower arrangements, like
are typically found at banquets and weddings. Due to her talent and dedication to doing a
good job, she was highly successful, and her business grew to where she now has a shop
located in a highly visible and successful strip mall. As with many successful businesses,
her success has produced unanticipated problems, some of which are normal growth
pains, but others are relatively unique to the type of business. At a recent meeting with her
business advisor, she outlined some of the major issues she faces:
1. Business Focus. When she moved into her new shop in the mall, she continued
to specialize in the make-to-order specialty arrangements, but customers frequently
16
Chapter one
walked into her shop requesting “spot” purchases, including gifts for sick friends
and last-minute flower purchases for occasions such as birthdays, anniversaries,
Valentine’s Day, and so forth. As this business represented an attractive addition to
the store revenue, she accommodated it with three large climate-controlled display
cases stocked with ready-to-sell arrangements of various sizes, types, and costs. Even
though she did not aggressively pursue this market with advertising, the heavy mall
traffic where her store is located and word of mouth caused the walk-in business to
steadily grow to where it now represents almost half of her total revenue. This business has brought her numerous headaches, however, due to several characteristics:
a. Even though some days have predictably high demand (e.g., just prior to Valentine’s
day, Mother’s day), most of the time she has no idea how many customers will come
in for spot buys an any given day, nor does she have any idea as to the price range
they will look for. Even such variables as the weather and the schedule of local
sports teams appear to affect her demand. She knows she needs to manage this demand better, because not having what a customer wants could mean the permanent
loss of a good potential customer. On the other side, flowers have a limited shelf
life, and having too much of the wrong price range could mean a high spoilage rate.
It would not take many lost sales on a daily basis to represent the difference between
profit and loss for that part of the business.
b. Some customers have become irate that her delivery system, a major part of the
make-to-order business, will not accommodate the delivery of a $20 ready-to-sell
arrangement to a hospital, for example. Angry customers have even asked her how
much they need to spend on an arrangement before she will deliver. She has never
really thought about an answer to that question and has not known how to reply.
Generally, she just states that she does not deliver premade flower arrangements.
She knows this lack of delivery has cost her some goodwill, some business, and
perhaps even some potential return customers.
c. Related to the point above, several customers have expressed serious dissatisfaction that she is not a member of some national delivery service, so they can have
flowers delivered out of town. She is afraid such a business will pull her even
further from her core business of make-to-order, as those services typically focus
on catalogs of set designs. As those services are also expensive to belong to, she
knows she would have to spend a lot more time and effort in that area to make it
financially feasible.
d. Another group of customers wants her to extend her open shop hours, as they say
they occasionally drop by for flowers on their way home from work and often find
her closed for the day or at least not available while she is setting up a flower order
in some other location.
2. Personnel Issues. As her business grew, Fran hired another skilled arranger, Molly,
to work with her. The unpredictability of the walk-in demand has caused her to bring
people issues up as a problem, however. As walk-in customers demand immediate
attention, she and Molly are frequently called to the front of the shop to sell arrangement from the cases. This pulls them away from working on their orders, and while
she has been late only on a couple of special orders within the last few weeks, several
others were delivered before she was satisfied with their appearance, merely to avoid
their being late. This worries her a great deal, as she has worked very hard to obtain
a reputation for the quality of her arrangements. She thought about hiring a delivery
person, but decided it was important that either she or Molly deliver the orders so that
they may put last-minute touches on the arrangement in case of disturbance during the
delivery process.
Instead she opted to hire some part-time unskilled help for the shop to handle the
walk-in shop sales. This has proved less than satisfactory, because of two reasons:
a. The unpredictability of demand has her constantly wondering about what hours
and how many hours to schedule the help. The extra help adds to overall cost, and
paying someone to stand around while no customers come in the shop makes the
difference between profit and loss even more sensitive.
Introduction to Materials Management
17
b. Customers frequently have questions about the type of flowers in an arrangement,
how long they last, and so forth. The unskilled workers she hires often don’t know
what to answer. They will then frequently interrupt either Fran or Molly with the
question, and even when they get the answer the customer often is left with a poor
impression, as they often expect more knowledge from a salesperson. The impression is even worse if both Fran and Molly are out servicing orders, as the only
answer the customer gets is "I’m not sure." Since she pays only slightly above
minimum wage, her turnover is high. This means she is constantly trying to hire
and train people, further distracting her from her main business. She knows she
could reduce the turnover and hire more knowledgeable people if she paid her help
more per hour, but that issue again pushes her closer to the loss column for many of
the days the shop is open, so she feels she really can’t afford to pay more.
3. Expansion. Several of her regular customers are encouraging her to open another
operation on the other side of the city, as well as considering expansion to other cities.
They claim several of their friends like her arrangements a great deal, but consider her
location too inconvenient from where they live or work. That is typically not a problem for large orders, as she or Molly will typically offer to visit the customer to obtain
details for the arrangement. That does take a lot of time, however, so she finds herself
more frequently asking the long distance customer to come to the shop if possible.
Many decline to do so, and the order is sometimes lost. While expansion is attractive
to her, she worries about control—not only for order servicing, but also for delivery.
How can she possibly maintain control of quality and design in two or more locations
at the same time?
4. Supply. As her purchases of flowers from the wholesaler has grown, the wholesaler
has recommended that Fran make a purchasing contract instead of making spot bulk
buys as she now does. This contract will give her significant quantity price discounts,
but her delivered quantity has to be above a certain amount of each type of flower so
that the wholesaler can reduce costs due to economies of scale. The quantities she
needs to order are reasonable given her average demand, but the fluctuation around
that average is large enough to present significant spoilage during certain periods. She
wonders if she would be better off in the long run with the purchasing contract.
assignment
1. What are the key issues in this case? In other words, analyze the case to try to determine the true problems from the symptoms of those problems. How do these issues
relate to the issue of strategy?
2. What type of data would you suggest collecting to both verify the problem identifications are correct and to provide solution approaches and support? How would you
organize and use that data?
3. What would you suggest she do with her business and why? Provide a comprehensive
and integrated plan of action and provide support for your suggestions.
4. Develop an implementation plan for whatever changes you suggest she make.
Prioritize the key steps if appropriate.
Chapter
two
Production Planning SyStem
introduction
This chapter introduces the manufacturing planning and control (MPC) system. First, it
deals with the total system and then with some details involved in production planning.
Subsequent chapters discuss master scheduling, material requirements planning, capacity
management, production activity control, purchasing, and forecasting.
Manufacturing is complex. Some firms make a few different products, whereas others
make many products. However, each uses a variety of processes, machinery, equipment,
labor skills, and material. To be profitable, a firm must organize all these factors to make
the right goods at the right time at top quality and do so as economically as possible. It is a
complex problem, and it is essential to have a good planning and control system.
A good planning system must answer the following four questions:
1. What are we going to make or provide to customers?
2. What does it take to make it?
3. What do we have?
4. What do we need?
These are questions of priority and capacity.
Priority relates to what products are needed, how many are needed, and when they
are needed. The marketplace establishes the priorities. Manufacturing is responsible for
devising plans to satisfy the market demand if possible.
Capacity is the capability of manufacturing to produce goods and services. Eventually it
depends on the resources of the company—the machinery, labor, and financial resources, and
the availability of material from suppliers. In the short term, capacity is the quantity of work
that labor and equipment can perform in a given period. The relationship that should exist
between priority and capacity is shown graphically in Figure 2.1.
In the long and short term, manufacturing must devise plans to balance the demands
of the marketplace with its resources and capacity. For long-range decisions, such as the
building of new plants or the purchase of new equipment, the plans must be made for several years. For planning production over the next few weeks, the time span will be days or
weeks. This hierarchy of planning, from long range to short range, is covered in the next
section.
PRIORITY
(Demand)
Figure 2.1 Priority–capacity relationship.
18
CAPACITY
(Resources)
Production Planning System
19
manufacturing Planning and control SyStem
There are five major levels in the manufacturing planning and control system:
Strategic business plan (a business plan based on the strategy).
Production plan (sales and operations plan).
■■ Master production schedule.
■■ Material requirements plan.
■■ Purchasing and production activity control.
■■
■■
Each level varies in purpose, time span, and level of detail. As the process moves from
strategic planning to production activity control, the purpose changes from general direction
to specific detailed planning, the time span decreases from years to days, and the level of
detail increases from general categories to individual components and workstations.
Since each level is for a different time span and for different purposes, each differs in
the following:
Purpose of the plan.
Planning horizon—the time span from now to some time in the future for which the
plan is created.
■■ Level of detail—the detail about products required for the plan.
■■ Planning cycle—the frequency with which the plan is reviewed.
■■
■■
At each level, three questions must be answered:
1. What are the priorities—how much of what is to be produced and when?
2. What is the available capacity—what resources do we have?
3. How can differences between priorities and capacity be resolved?
Figure 2.2 shows the planning hierarchy. The first four levels are planning levels.
The result of the plans is authorization to purchase or manufacture what is required.
The final level is when the plans are put into action through purchasing and production
activity control.
STRATEGIC
BUSINESS
PLAN
PRODUCTION
PLAN
MASTER
PLAN
MASTER
PRODUCTION
SCHEDULE
PLANNING
MATERIAL
REQUIREMENTS
PLAN
PRODUCTION
ACTIVITY CONTROL
AND
PURCHASING
IMPLEMENTATION
Figure 2.2 Manufacturing planning and control system.
20
Chapter two
The following sections will examine each of the planning levels by purpose, horizon,
level of detail, and planning cycle.
The Strategic Plan
The strategic plan of a firm is a statement of the major goals and objectives the company
expects to achieve over the next 2 to 10 years or more. It is a statement of the broad direction of the firm and shows the kind of business—product lines, markets, and so on—the
firm wants to do in the future. The plan gives general direction about how the company
hopes to achieve these objectives and really represents a commitment to take various
actions designed toward growth, defining and attracting customers, defining markets,
and improving competitive and financial performance. It is based on long-range forecasts
and includes participation from marketing, finance, production, engineering and all other
major functions in the firm. In turn, the plan provides direction and coordination among
the marketing, production, financial, engineering, and other functional plans.
Marketing and Sales are responsible for analyzing the marketplace and deciding the
firm’s response: the markets to be served, the products supplied, desired levels of customer service, pricing, promotion strategies, and so on.
Finance is responsible for deciding the sources and uses of funds available to the
firm, cash flows, profits, return on investment, and budgets.
Production must satisfy the demands of the marketplace. It does so by using plants,
machinery, equipment, labor, and materials as efficiently as possible.
Engineering is responsible for research, development, and design of new products
or modifications to existing ones. Engineering must work with marketing and production
to produce designs for products that will sell in the marketplace and can be made most
economically.
The development of the strategic plan is the responsibility of senior management.
Using information from marketing, finance, production, and other functions the strategic
plan provides a framework that sets the goals and objectives for further planning by the
marketing, finance, engineering, and production departments.
Some companies have adopted a special approach of establishing vision statements
and goals as part of the planning process. Tactical work plans are established for the
visions to allow all parts of the organization to move systematically toward the overall
goal. Developed by Japanese companies, the approach is often given its Japanese name
Hoshin Planning. The basic steps include:
1. Making the plan for what you wish to improve or accomplish.
2. Establishing subgoals.
3. Communicating the plan in the organization.
4. Measuring your results.
5. Analyzing data from the measures and taking corrective action as needed.
6. Repeating as necessary.
There are recent trends in business that sometimes impact the development and management of strategic plans. One of these is the issue of sustainability, basically meaning
the capability to continue (sustain) operations into the long term. Much of the interest in
sustainability evolved from the perspective of pollution control, saving the environment
and social responsibility (establishing company policies that establish a positive relationship with society and strike some balance between the economy and the environment).
Corporate social responsibility has become an important issue worldwide, as indicated by
the United Nations Global Compact. This compact recognizes that business is a primary
source of globalization, and the compact lays out ten principles for business strategy and
operations to maintain appropriate human rights, treatment of labor, environmental issues,
and anticorruption principles.
Sustainability is also based on the concepts of reduction of waste and inefficiency in
production, leading not only to using fewer resources and producing less waste, but also less
costs. Examples of waste reduction might include less need for packaging (often thrown
Production Planning System
21
away) and using resources to produce reusable outputs. These concepts are explained in
much more detail in Chapter 15 (Lean Production). Recycling and reusing material is also
a major part of sustainability. Sometimes this activity is described as remanufacturing
or reverse logistics. In some cases companies will establish a formal supply chain used
to retrieve a used product in order to dispose of it, reclaim materials from it, or reuse it in
some fashion. This is sometimes referred to as a reverse supply chain.
A second recent development impacting strategic planning is risk management.
Often people consider risk to be a negative issue, and of course that is partially correct.
Risks reflecting problems from some sort of failure of systems, people, or external events
can result in loss of money, productivity, legal problems, and a reduction in the probability
of successful implementation of the strategic plan. Risks can also be positive, and in this
context are often called opportunities. Risk management is focused on establishing systems
and measurements to try to quickly recognize risks and establish strategic mechanisms to
minimize impacts from negative risks and take advantage of positive risks (opportunities).
Effective strategic planning depends on obtaining appropriate measurements and
feedback on how well the tactics related to the plan are working toward the overall set
of strategic goals for the organization. These measures (both financial and nonfinancial)
are sometimes referred to as Key Performance Indicators (KPI). It is important that
one measure or subset of measures do not contradict with others—that they provide an
overall balance in indicating the progress of the company toward the overall strategic
plan and sustainability efforts including financial, societal, and environmental goals. The
set of KPIs that are balanced are often referred to as a balanced scorecard, and detailed
approaches have been developed to both establish and manage balanced scorecards. The
scorecard tends to balance measures dealing with business processes, financial measures,
customer focused measures, and learning and growth. All these perspectives and measures
are, of course, developed as part of the overall strategic planning process.
The Strategic Business Plan (Business Plan)
Once the strategic plan has been established, the plan is often restated in financial terms,
including projected revenues, a projected balance sheet, and a projected income statement.
This financially based plan is often called the business plan or sometimes the strategic
business plan. Each department produces its own plans to achieve the objectives set by
the strategic business plan. These plans will be coordinated with one another and with the
strategic business plan. Figure 2.3 shows this relationship.
Production
Plan
Financial
Plan
STRATEGIC
BUSINESS
PLAN
Engineering
Plan
Figure 2.3 Business plan.
Marketing
Plan
22
Chapter two
The level of detail in the strategic business plan is not high. It is concerned with
general market and production requirements—total market for major product groups or
product families, perhaps—and not sales of individual items. It is usually stated in dollars
rather than units.
Strategic business plans are usually reviewed every six months to a year.
The Production Plan
Given the objectives set by the strategic business plan, production management is concerned with the following:
The quantities of each product group that must be produced in each period.
The desired inventory levels.
■■ The resources of equipment, labor, and material needed in each period.
■■ The availability of the resources needed.
■■
■■
The level of detail of the production plan is not high. For example, if a company
manufactures children’s bicycles, tricycles, and scooters in various models, each with
many options, the production plan will show major product groups, or families: bicycles,
tricycles, and scooters. Because the production plan tends to combine product groups
or product families rather than individual products, it is sometimes referred to as the
aggregate production plan.
Production planners must devise a plan to satisfy market demand within the resources
available to the company. This will involve determining the resources needed to meet
market demand, comparing the results to the resources available, and devising a plan to
balance requirements and availability.
This process of determining the resources required and comparing them with the available resources takes place at each of the planning levels and is the purpose of capacity management. For effective planning, there must be a balance between priority and capacity.
Along with the market and financial plans, the production plan is concerned with
implementing the strategic business plan. The planning horizon is usually 6 to 18 months
and is usually reviewed each month or quarter.
The Master Production Schedule
The master production schedule (MPS) is a plan for the production of individual end
items. It breaks down the production plan to show, for each period, the quantity of each
end item to be made. For example, it might show that 200 Model A23 scooters are to be
built each week. Inputs to the MPS are the production plan, the forecast for individual end
items, sales orders, inventories, and existing capacity.
The level of detail for the MPS is greater than for the production plan. Whereas the
production plan was based upon families of products (e.g., tricycles), the master production schedule is developed for individual end items (each model of tricycle). The planning
horizon usually extends from 3 to 18 months but primarily depends on the purchasing and
manufacturing lead times. This is discussed in Chapter 3 in the section on master scheduling. The term master scheduling describes the process of developing a master production
schedule. The term master production schedule is the end result of this process. Usually,
the plans are reviewed and changed weekly or monthly.
The Material Requirements Plan
The material requirements plan (MRP) is a plan for the production and purchase
of the components used in making the items in the master production schedule. It shows
the quantities needed and when manufacturing intends to make or use them. Purchasing
and production activity control use the MRP to execute the purchase or manufacture of
specific items.
The level of detail of MRP is high. The material requirements plan establishes when
the components and parts are needed to make each end item.
Production Planning System
23
PAC
LEVEL OF DETAIL
MRP
MPS
Production
Plan
Strategic
Business
Plan
PLANNING HORIZON (Time)
Figure 2.4 Level of detail versus planning horizon.
The planning horizon is at least as long as the combined purchase and manufacturing lead
times. As with the master production schedule, it usually extends from 3 to 18 months.
Purchasing and Production Activity Control
Purchasing and production activity control (PAC) represent the implementation and
control phase of the production planning and control system. Purchasing is responsible for
establishing and controlling the flow of raw materials into the factory. PAC is responsible
for planning and controlling the flow of work through the factory.
The planning horizon is very short, perhaps from a day to a month. The level of detail
is high since it is concerned with individual components, workstations, and orders. Plans
are reviewed and revised daily.
Figure 2.4 shows the relationship among the various planning tools, planning horizons, and level of detail.
This chapter focuses on production planning. Later chapters deal with master scheduling, material requirements planning, purchasing and production activity control.
Capacity Management
At each level in the manufacturing planning and control system, the priority plan must be
tested against the available resources and capacity of the manufacturing system. Chapter 5
describes some of the details of capacity management. For now, it is sufficient to understand that the basic process in capacity management is one of calculating the capacity
needed to manufacture per the requirements of the priority plan and of finding methods
to make that capacity available. There can be no valid, workable production plan unless
this is done. If the capacity cannot be made available when needed, then the plans must be
changed.
Determining the capacity required, comparing it to available capacity, and making
adjustments (or changing plans) must occur at all levels of the manufacturing planning
and control system.
Over several years, machinery, equipment, and plants can be added to or taken away
from manufacturing. Some changes, such as changing the number of shifts, working overtime, subcontracting the work, and so on, can be accomplished in these time spans. However,
in the time spans involved from production planning to production activity control, these
kinds of large changes cannot be made.
SaleS and oPerationS Planning
The strategic business plan integrates the plans of all the departments in an organization and is normally updated annually. However, these plans should be updated as time
progresses so that the latest forecasts and market and economic conditions are taken into
24
Chapter two
STRATEGIC
BUSINESS PLAN
ANNUAL
SALES AND OPERATIONS PLAN
MARKETING
PLAN
PRODUCTION
PLAN
DETAILED SALES
PLAN
MASTER
PRODUCTION
SCHEDULE
MONTHLY
WEEKLY OR
DAILY
Figure 2.5 Sales and operations planning.
account. Sales and operations planning (S&OP, or sometimes just SOP) is a process
for continually revising the strategic business plan and coordinating plans of the various
departments. SOP is a cross-functional business plan that involves sales and marketing,
product development, operations, and senior management. Operations represent supply,
whereas marketing represents demand. The SOP is the forum in which the production plan
is developed.
Although the strategic plan and strategic business plan are usually updated annually,
sales and operations planning is a dynamic process in which the company plans are updated
on a regular basis, usually at least monthly. The process starts with the sales and marketing
departments, which compare actual demand with the sales plan, assess market potential, and
forecast future demand. The updated marketing plan is then communicated to manufacturing,
engineering, and finance, which adjust their plans to support the revised marketing plan. If
these departments find they cannot accommodate the new marketing plan, then the marketing
plan must be adjusted. In this way the strategic business plan is continually revised throughout the year and the activities of the various departments are coordinated. Figure 2.5 shows
the relationship between the strategic business plan and the sales and operations plan.
Sales and operations planning is medium range and includes the marketing, production, engineering, and finance plans. Sales and operations planning has several benefits:
It provides a means of updating the strategic plan and the strategic business plan as
conditions change.
■■ It provides a means of managing change. Rather than reacting to changes in market
conditions or the economy after they happen, the SOP forces management to look at
the economy at least monthly and places it in a better position to plan changes.
■■ It ensures that the various department plans are realistic and coordinated and support
the business plan.
■■ It provides a realistic plan that can achieve the company objectives.
■■ It permits better management of production, inventory, and backlog (i.e., unfilled
customer orders).
■■
It is important to realize that effective sales and operations planning is an executivelevel planning process that involves the potential to make significant trade-offs between
the various functions/departments in the company. In this way executives involved in
planning can ensure that the best approach to volume and mix are made and that supply
and demand are balanced in the best approach possible.
Production Planning System
25
It should be noted that the sales and operations plan does not actually schedule production, but instead focuses on producing a high-level plan for the use of company resources.
These resources include not only production resources but also human resources, sales
resources, financial resources, and virtually all other functions in the company. The sales and
operations plan should reflect the vision and strategies developed in the strategic plan, and
should serve as a focused, single plan across functional areas that provides the approach to
running the entire business.
In his book Sales and Operations Planning, author Tom Wallace describes a five-step
process in developing the S&OP, summarized here:
1. Data gathering. Actual past month sales, existing inventory levels, marketing/sales
data, financial data, and so on.
2. Demand planning. Using data and other inputs from all appropriate sources to establish management forecasts. Statistical forecasts can be run, but should be evaluated in
the context of other inputs. Those inputs may include new product introductions, price
change impacts, competitive movements, and economic conditions. Demand planning
and demand management is more fully described in Chapter 8 of this book.
3. Supply planning. Comparison of demand forecasts with capacity constraints.
4. A pre-S&OP meeting. This meeting should be used for balancing supply and
demand, resolving differences (if possible), and developing action recommendations
and an agenda for the top manager S&OP meeting.
5. The executive meeting. The final decisions are made as to how the company should
proceed given all the data, recommendations, and how well the plan fits into the context of the strategic plan and the strategic business plan.
In many companies the resource discussions include some aspects of what is often
called green production. Basically these discussions will focus on issues such as
Environmental impacts.
Energy conservation.
■■ Material usages, such as waste reduction, reuse and recyclability.
■■ Scarcity of various resources.
■■
■■
The conclusions from these green resource discussions can help in both the design of
products and process and in the most effective uses of production resources. As an example, a metal casting manufacturer in Canada, which uses a lot of electricity, schedules its
melts to avoid periods of peak demand. This avoids the local energy supplier from having
to increase their capacity and in turn the manufacturer is given a slightly preferred rate.
It should be obvious that the concepts under the category of green production are a major
part of sustainability discussed earlier in this chapter.
manufacturing reSource Planning
Because of the large amount of data and the number of calculations needed, the manufacturing planning and control system will probably have to be computer based. If a computer is not used, the time and labor required to make calculations manually is extensive
and forces a company into compromises. Instead of scheduling requirements through the
planning system, the company may have to extend lead times and build inventory to compensate for the inability to schedule quickly what is needed and when.
The system is intended to be a fully integrated planning and control system that
works from the top down and has feedback from the bottom up. Strategic business planning integrates the plans and activities of marketing, finance, and production to create
plans intended to achieve the overall goals of the company. In turn, master production
scheduling, material requirements planning, production activity control, and purchasing
are directed toward achieving the goals of the production and strategic business plans and,
ultimately, the company. If priority plans have to be adjusted at any of the planning levels
because of capacity problems, those changes should be reflected in the levels above. Thus,
there must be feedback throughout the system.
Chapter two
The strategic business plan incorporates the plans of marketing, finance, and production. Marketing must agree that its plans are realistic and attainable. Finance must agree
that the plans are desirable from a financial point of view, and production must agree
that it can meet the required demand. The manufacturing planning and control system,
as described here, is a master game plan for all departments in the company. This fully
integrated planning and control system is called a manufacturing resource planning, or
MRP II system. The term MRP II is used to distinguish the manufacturing resource plan
(MRP II) from the material requirements plan (MRP).
MRP II provides coordination between marketing and production. Marketing, finance,
and production agree on a total workable plan expressed in the production plan. Marketing
and production must work together on a weekly and daily basis to adjust the plan as
changes occur. Order sizes may need to be changed, orders canceled, and delivery dates
adjusted. This kind of change is made through the master production schedule. Marketing
managers and production managers may change master production schedules to meet
changes in forecast demand. Senior management may adjust the production plan to reflect
overall changes in demand or resources. However, they all work through the MRP II system. It provides the mechanism for coordinating the efforts of marketing, finance, production, and other departments in the company. MRP II is a method for the effective planning
of all resources of a manufacturing company.
Note the feedback loops in the MRP II system shown in Figure 2.6.
BUSINESS PLAN
RESOURCES OK?
No
Yes
SALES PLAN
FEEDBACK
SALES AND OPERATION PLAN
MARKETING
PRODUCTION
PLAN
PLAN
Closed-Loop MRP
MASTER SCHEDULE
MATERIAL REQUIREMENTS PLAN
RESOURCES OK?
No
FEEDBACK
26
Yes
PURCHASING
PRODUCTION
ACTIVITY
CONTROL
PERFORMANCE MEASURES
Figure 2.6 Manufacturing resource plan (MRP II).
Production Planning System
27
enterPriSe reSource Planning
As MRP systems evolved, they tended to take advantage of two changing conditions:
1. Computers and information technologies (IT) becoming significantly faster, more
reliable, and more powerful. People in most companies had become at least comfortable, but often very familiar, with the advantages in speed, accuracy, and capability of
integrated computer-based management systems.
2. Movement toward integration of knowledge and decision making in all aspects
of direct and indirect functions and areas that impact materials flow and materials
management. This integration not only included internal functions such as marketing, engineering, human resources, accounting, and finance but also the upstream
activities in supplier information and the downstream activities of distribution and
delivery. That movement of integration is what is now recognized as supply chain
management.
As the needs of the organization grew in the direction of a truly integrated approach
toward materials management, the development of IT systems matched that need. As these
systems became both larger in scope and integration when compared to the existing MRP
and MRP II systems, they were given a new name—enterprise resource planning (ERP).
ERP is similar to the MRP II system except it does not dwell on manufacturing. The
whole enterprise is taken into account. APICS Dictionary, 14th Edition defines ERP as a
“framework for organizing, defining, and standardizing the business processes necessary
to effectively plan and control an organization so the organization can use its internal
knowledge to seek external advantage.” To fully operate, there must be applications for
planning, scheduling, costing, and so forth, for all layers in an organization, work centers,
sites, divisions, and corporate. Essentially, ERP encompasses the total company, whereas
MRP II encompasses just manufacturing. The larger scope of ERP systems allows the
tracking of orders and other important planning and control information throughout the
entire company, from procurement to ultimate customer delivery. In addition, many ERP
systems are capable of allowing managers to share data between firms, meaning that these
managers can potentially have visibility across the complete span of the supply chain.
Although the power and capability of these highly integrated ERP systems is extremely
high, there are also some large costs involved. Many of the best systems are expensive to
buy. The large data requirements (for both quantity and accuracy) tend to make the systems
expensive, time consuming, and for many firms, generally difficult to implement.
As the concept in supply chain grew, another planning approach was developed.
Called advanced planning and scheduling (APS) systems, the approach has often
included the suppliers and customers in the planning, thereby attempting to optimize the
performance of the entire supply chain. Extracting information from the entire supply
chain, the system attempts to create a rapid and feasible schedule for satisfying customer
demand. It includes mathematical optimization and analytic tools and the principle of
finite scheduling (see Chapter 6). These same concepts can also be used internally in an
operation of a single company in order to try to optimize or create a more feasible solution
for the customers of that operation.
making the Production Plan
Thus far the purpose, planning horizon, and level of detail found in a production plan have
been discussed. This section includes some details involved in making production plans.
Based on the market plan and available resources, the production plan sets the limits
or levels of manufacturing activity for some time in the future. It integrates the capabilities
and capacity of the factory with the market and financial plans to achieve the overall business goals of the company.
The production plan sets the general levels of production and inventories over the
planning horizon. Its prime purpose is to establish production rates that will accomplish
the objectives of the strategic plan and the strategic business plan. These include inventory levels, backlogs, market demand, customer service, low-cost plant operation, labor
28
Chapter two
relations, and so on. The plan must extend far enough in the future to plan for the labor,
equipment, facilities, and material needed to accomplish it. Typically, this is a period of
6 to 18 months and is done in monthly and sometimes weekly periods.
The planning process at this level ignores such details as individual products, colors,
styles, or options. With the time spans involved and the uncertainty of demand over long
periods, the detail would not be accurate or useful, and the plan would be expensive to create. For planning purposes, a common unit or small number of product groups is what is
needed.
Establishing Product Groups
Firms that make a single product, or products that are similar, can measure their output
directly by the number of units they produce. A brewery, for instance, might use barrels of
beer as a common denominator. Many companies, however, make several different products, and a common denominator for measuring total output may be difficult or impossible
to find. Product groups or families need to be established. Marketing naturally looks at
products from the customers’ point of view of functionality and application, whereas manufacturing looks at products in terms of processes. Thus, firms need to establish product
groups based on the similarity of manufacturing processes and resources used.
Manufacturing must provide the capacity to produce the goods needed. It is concerned more with the demand for the specific kinds of capacity needed to make the products than with the demand for the product.
Capacity is the ability to produce goods and services. It means having the resources
available to satisfy demand. For the time span of a production plan, it can be expressed as
the time available or, sometimes, as the number of units or dollars that can be produced
in a given period. The demand for goods must be translated into the demand for capacity.
At the production planning level, where little detail is needed, this requires identifying
product groups, or families, of individual products based on the similarity of manufacturing process. For example, several calculator models might share the same processes and
need the same kind of capacity, regardless of the variations in the models. They would be
considered as a family group of products.
Over the time span of the production plan, large changes in capacity are usually not
possible. Additions or subtractions in plant and equipment are impossible or very difficult to
accomplish in this period. However, some things can be altered, and it is the responsibility of
manufacturing management to identify and assess them. Usually the following can be varied:
People can be hired and laid off, overtime and short time can be worked, and shifts
can be added or removed.
■■ Inventory can be built up in slack periods and sold or used in periods of high demand.
■■ Work can be subcontracted or extra equipment leased.
■■
Each alternative has its associated benefits and costs. Manufacturing management is
responsible for finding the least-cost alternative consistent with the goals and objectives
of the business.
Basic Strategies
In summary, the production planning problem typically has the following characteristics:
A time horizon of 12 months or more is used, with periodic updating perhaps every
month or quarter.
■■ Production demand consists of one or a few product families or common units.
■■ Demand is fluctuating or seasonal.
■■ Plant and equipment are fixed within the time horizon.
■■ A variety of management objectives are set, such as low inventories, efficient plant
operation, good customer service, and good labor relations.
■■
Figure 2.7 shows a hypothetical demand forecast for a product group. Note that the
demand is seasonal.
29
DEMAND
Production Planning System
Demand
J
F
M
A
M
J
J
A
S
O
N
D
TIME
Figure 2.7 Hypothetical demand curve.
There are four basic strategies that can be used in developing a production plan:
1. Chase strategy.
2. Production leveling.
3. Subcontracting.
4. Hybrid strategy.
DEMAND
Chase (demand matching) strategy Chase strategy means producing the amounts
demanded at any given time. Inventory levels remain stable while production varies to
meet demand. Figure 2.8 shows this strategy. The firm manufactures just enough at any
one time to meet demand exactly. In some industries, this is the only strategy that can
be followed. Farmers, for instance, must produce in the growing season. The post office
must process mail over the Christmas rush and in slack seasons. Restaurants have to serve
meals when the customers want them. These industries cannot stockpile or inventory their
products or services and must be capable of meeting demand as it occurs.
In these cases, the company must have enough capacity to be able to meet the peak
demand. Farmers must have sufficient machinery and equipment to harvest in the growing season, although the equipment will lie idle in the winter. Companies with seasonal
or cyclical demand often have to hire and train people for the peak periods and lay them
off when the peak is past. Sometimes they have to add extra shifts and overtime. All these
changes add cost.
The advantage to the chase strategy is that inventories can be kept to a minimum. Goods
are made when demand occurs and are not stockpiled. Thus, the costs associated with carrying inventories are avoided. Such costs can be quite high, as is shown in Chapter 9,
on inventory fundamentals.
Demand
Production
J
F
M
A
M
J
TIME
Figure 2.8 Chase (demand matching) strategy.
J
A
S
O
N
D
Chapter two
DEMAND
30
Demand
Production
J
F
M
A
M
J
J
A
S
O
N
D
TIME
Figure 2.9 Production leveling strategy.
Production leveling Production leveling is continually producing an amount equal
to the average demand. This relationship is shown in Figure 2.9. Companies calculate
their total demand over the time span of the plan and, on the average, produce enough to
meet it. Sometimes demand is less than the amount produced and an inventory builds up.
At other times demand is greater and inventory is used up.
The advantage of a production leveling strategy is that it results in a smooth level of
operation that avoids the costs of changing production levels. Firms do not need to have
excess capacity to meet peak demand. They do not need to hire and train workers and lay
them off in slack periods. They can build a stable workforce. The disadvantage is that inventory will build up in low-demand periods, which will cost money to carry. In addition, an
understated forecast could result in not enough inventory being produced for the peak season.
Production leveling means the company will use its resources at a uniform rate and
produce the same amount each day it is operating. The exact amount produced each month
(or sometimes each week), however, will not be constant because the number of working
days varies from month to month.
example Problem
A company wants to produce 10,000 units of an item over the next three months at a
uniform rate. Because of an annual shutdown in the third month, the first month has
20 working days; the second, 21 working days; and the third, 12 working days. On the
average, how much should the company produce each day to level production?
Answer
Total production = 10,000 units
Total working days = 20 + 21 + 12 = 53 days
10,000
Average daily production =
= 188.7 units
53
For some products for which demand is very seasonal, such as Christmas tree lights,
some form of production leveling strategy is necessary. The costs of idle capacity, and of
hiring, training, and laying off, would be severe if a company employed a chase strategy.
Subcontracting As a pure strategy, production leveling would mean always producing at the defined level. Any demand above that level, assuming inventory is not available,
would imply the demand would not be met or meeting the extra demand by some other
method. A common production choice in those cases is meeting any additional demand
through subcontracting. Subcontracting can mean buying the extra amounts demanded
from external sources. An alternate choice is to purposely turn away extra demand. The
latter can be done by increasing prices when demand is high or by extending lead times.
This situation is shown in Figure 2.10.
The major advantage of these approaches is production cost. Costs associated with
excess production capacity are avoided, and because production is leveled, there are no
31
DEMAND
Production Planning System
Demand
Production
J
F
M
A
M
J
J
A
S
O
N
D
TIME
Figure 2.10 Subcontracting.
costs associated with changing production levels. The main disadvantage of subcontracting
is that the cost of purchasing (item cost, purchasing, transportation, and inspection costs)
may be greater than if the item were made in the plant. Subcontracting may also be used as
part of a chase strategy.
Few companies make everything or buy everything they need. The decision about
which items to buy and which to manufacture depends mainly on cost, but there are several other factors that may be considered.
Firms may manufacture to keep confidential processes within the company, to maintain quality levels, and to maintain a workforce.
They may buy from a supplier who has special expertise in design and manufacture
of a component, to allow the firm to concentrate on its own area of expertise, or to provide
known and competitive prices. This is discussed further in Chapter 14.
For many items, such as nuts and bolts or components that the firm does not normally
make, the decision is clear. For other items that are within the specialty of the firm, a decision must be made whether to subcontract or not.
DEMAND
Hybrid strategy The three strategies discussed so far are pure strategies. Each has its
own set of costs: equipment, hiring/layoff, overtime, inventory, and subcontracting. In reality, there are many possible hybrid or combined strategies a company may use. Each will
have its own set of cost characteristics. Production management is responsible for finding
the combination of strategies that minimizes the sum of all costs involved, providing the
level of service required, and meeting the objectives of the financial and marketing plans.
Figure 2.11 shows a possible hybrid plan. Demand is matched to some extent, production is partially smoothed, and in the peak period some subcontracting takes place. The
plan is only one of many that could be developed.
Demand
Production
J
F
M
A
M
J
TIME
Figure 2.11 Hybrid strategy.
J
A
S
O
N
D
32
Chapter two
Developing a Make-to-Stock Production Plan
In a make-to-stock environment, products are made and put into inventory before an order
is received from a customer. Sale and delivery of the goods are made from inventory. Offthe-rack clothing, frozen foods, and bicycles are examples of this kind of manufacturing.
Generally firms make to stock when
Demand is fairly constant and predictable.
There are few product options.
■■ Delivery times demanded by the marketplace are much shorter than the time needed to
make the product.
■■ Product has a long shelf life.
The information needed to make a production plan is as follows:
■■ Forecast by period for the planning horizon.
■■ Opening inventory.
■■ Desired ending inventory.
■■ Any past-due customer orders. These are orders that are late for delivery and are sometimes called backorders. Note the difference between the terms backorders and backlogs (mentioned earlier in the chapter). The backlog is customer orders received but not
yet shipped, while the backorders are customer orders that may be past due or due for
immediate shipment but cannot because the inventory levels are too low to fill the order.
■■
■■
The objective in developing a production plan is to minimize the costs of carrying
inventory, changing production levels, and stocking out (not having the products the customer wants when they are wanted).
The following sections develop a plan for leveling production and one for chase strategy.
Level production plan Following is the general procedure for developing a plan for
level production.
1. Total the forecast demand for the planning horizon.
2. Determine the opening inventory and the desired ending inventory.
3. Calculate the total production required as follows:
Total production = Total forecast + back orders
+ ending inventory − opening inventory
4. Calculate the production required each period by dividing the total production by the
number of periods.
5. Calculate the ending inventory for each period.
example Problem
Amalgamated Fish Sinkers makes a product group of fresh fish sinkers and wants to
develop a production plan for them. The expected opening inventory is 100 cases,
and the company wants to reduce that to 80 cases by the end of the planning period.
The number of working days is the same for each period. There are no backorders. The
expected demand for the fish sinkers is as follows:
Period
1
2
3
4
5
Total
Forecast (cases)
110
120
130
120
120
600
a. How much should be produced each period?
b. What is the ending inventory for each period?
c. If the cost of carrying inventory is $5 per case per period based on ending inventory,
what is the total cost of carrying inventory?
d. What will be the total cost of the plan?
Production Planning System
Period
33
1
2
3
4
5
Total
Forecast (cases)
110
120
130
120
120
600
Production
116
116
116
116
116
580
106
102
88
84
80
Ending
Inventory
100
Figure 2.12 Level production plan: Make-to-stock.
Answer
a. Total production required = 600 +■80 − 100 = 580 cases
580
Production each period =
= 116 cases
5
b. Ending inventory = opening inventory + production − demand
Ending inventory after the first period = 100 + 116 − 110 = 106 cases
Similarly, the ending inventories for each period are calculated as shown in
Figure 2.12. The ending inventory for period 1 becomes the opening inventory for
period 2:
Ending inventory (period 2) = 106 + 116 − 120 = 102 cases
c. The total cost of carrying inventory would be
(106 + 102 + 88 + 84 + 80) $5 = $2300
d. Since there were no stockouts and no changes in the level of production, this
would be the total cost of the plan.
Chase strategy Amalgamated Fish Sinkers makes another line of product called fish
stinkers. Unfortunately, they are perishable, and the company cannot build inventory for
sale later. They must use a chase strategy and make only enough to satisfy demand in each
period. Inventory costs will be a minimum, and there should be no stockout costs. However,
there will be costs associated with changing production levels.
Suppose in the preceding example that changing the production level by one case
costs $20. For example, a change from 50 to 60 would cost
160 - 502 * $20 = $200
The opening inventory is 100 cases, and the company wishes to bring this down to
80 cases in the first period. The required production in the first period would then be
110 - 1100 - 802 = 90 cases
Assuming that production in the period before period 1 was 100 cases, Figure 2.13
shows the changes in production levels and in ending inventory.
The cost of the plan would be
Cost of changing production level = 1602$20 = $1200
Cost of carrying inventory = (80 cases)(5 periods)$5 = $2000
Total cost of the plan = $1200 + $2000 = $3200
It should be noted that the previous examples provide a basic understanding of the
dollar cost of possible production plans. But before a final selection of a plan is made,
there are other considerations that should be evaluated, and some of those are difficult to
estimate financially. Some of those issues might include
Impact on customers if cyclical demand in a level schedule causes shortages.
Impact on production workers as they are moved into and out of production in a chase
strategy. Such an impact will often reduce efficiency of the operation.
■■ Potential loss of profit if customers change buying preferences and seek a competitor’s
products.
■■
■■
34
Chapter two
Period
0
Demand
(cases)
Production
100
Change in
Production
Ending
Inventory
100
1
2
3
4
5
Total
110
120
130
120
120
600
90
120
130
120
120
580
10
30
10
10
0
60
80
80
80
80
80
Figure 2.13 Chase strategy: Make-to-stock.
Developing a Make-to-Order Production Plan
In a make-to-order environment, manufacturers wait until an order is received from a
customer before starting to make the goods. Examples of this kind of manufacture are
custom-tailored clothing, machinery, or any product made to customer specification. Very
expensive items are usually made to order. Generally, firms make to order when
Goods are produced to customer specification.
■■ The customer is willing to wait while the order is being made.
■■ The product is expensive to make and to store.
■■ Several product options are offered.
■■
Assemble-to-order Where several product options exist, such as in automobiles, and
where the customer is not willing to wait until the product is made, manufacturers produce
and stock standard component parts. When manufacturers receive an order from a customer, they assemble the component parts from inventory according to the order. Since
the components are stocked, the firm needs only time to assemble before delivering to the
customer. Examples of assemble-to-order products include automobiles and computers.
Assemble-to-order is a subset of make-to-order.
The following information is needed to make a production plan for make-to-order
products:
Forecast by period for the planning horizon.
Opening backlog of customer orders.
■■ Desired ending backlog.
■■
■■
Backlog In a make-to-order environment, a company does not build an inventory of
finished goods. Instead, it has a backlog of unfilled customer orders. The backlog normally will be for delivery in the future and does not represent orders that are late or past
due. A custom woodwork shop might have orders from customers that will keep it busy
for several weeks. This will be its backlog. If individuals want some work done, the order
will join the queue or backlog. Manufacturers like to control the backlog so that they can
provide a good level of customer service.
Level production plan Following is a general procedure for developing a make-toorder level production plan:
1. Total the forecast demand for the planning horizon.
2. Determine the opening backlog and the desired ending backlog.
3. Calculate the total production required as follows:
Total production = total forecast + opening backlog - ending backlog
Production Planning System
35
Period
1
2
3
4
5
Total
Sales Forecast
100
100
100
100
100
500
Planned Production
104
104
104
104
104
520
Projected
Backlog
96
92
88
84
80
100
Figure 2.14 Level production plan: Make-to-order.
4. Calculate the production required each period by dividing the total production by the
number of periods.
5. Spread the existing backlog over the planning horizon according to due date per period.
example Problem
A local printing company provides a custom printing service. Since each job is different,
demand is forecast in hours per week. Over the next five weeks, the company expects
that demand will be 100 hours per week. There is an existing backlog of 100 hours,
and at the end of five weeks, the company wants to reduce that to 80 hours. How many
hours of work will be needed each week to reduce the backlog? What will be the backlog
at the end of each week?
Answer
Total production = 500 + 100 - 80 = 520 hours
520
= 104 hours
5
The backlog for each week can be calculated as
Weekly production =
Projected backlog = old backlog + forecast - production
For week 1: Projected backlog = 100 + 100 - 104 = 96 hours
For week 2: Projected backlog = 96 + 100 - 104 = 92 hours
Figure 2.14 shows the resulting production plan.
Resource Planning
Once the preliminary production plan is established, it must be compared to the existing
resources of the company. This step is called resource requirements planning or resource
planning. Two questions must be answered:
1. Are the resources available to meet the production plan?
2. If not, how will the difference be reconciled?
If enough capacity to meet the production plan cannot be made available, the plan
must be changed.
A tool often used is the resource bill. This shows the quantity of critical resources
(materials, labor, and bottleneck operations) needed to make one average unit of the
product group. Figure 2.15 shows an example of a resource bill for a company that makes
tables, chairs, and stools as a three-product family.
If the firm planned to make 500 tables, 300 chairs, and 1500 stools in a particular
period, they could calculate the quantity of wood and labor that will be needed. For example,
the amount of wood needed is
Tables :
500 * 20 = 10,000 board feet
Chairs :
300 * 10 = 3000 board feet
Stools :
1500 * 5 = 7500 board feet
Total wood required = 20,500 board feet
36
Chapter two
Product
Wood
(board
feet)
Labor
(standard
hours)
Tables
Chairs
Stools
20
10
5
1.31
0.85
0.55
Figure 2.15 Resource bill.
The amount of labor needed is
Tables :
500 * 1.31 = 655 standard hours
Chairs :
300 * 0.85 = 255 standard hours
Stools :
1500 * 0.55 = 825 standard hours
Total labor required = 1735 standard hours
The company must now compare the requirements for wood and labor with the availability of these resources. For instance, suppose the labor normally available in this period
is 1600 hours. The priority plan requires 1735 hours, a difference of 135 hours, or about
8.4%. Extra capacity must be found, or the priority plan must be adjusted. In this example,
it might be possible to work overtime to provide the extra capacity required. If overtime
is not possible, the plan must be adjusted to reduce the labor needed. This might involve
shifting some production to an earlier period or delaying shipments.
Summary
Production planning is the first step in a manufacturing planning and control system. The
planning horizon usually extends for a year. The minimum horizon depends on the lead
times to purchase materials and make the product. The level of detail is not high. Usually,
the plan is made for families of products based on the similarity of manufacturing processes or on some common unit. The production plan is part of the sales and operations
planning process, which is an executive-level planning process involving trade-offs across
departments or functions in the company.
Three basic strategies can be used to develop a production plan: chase, leveling production, or hybrid. Each has its operational and cost advantages and disadvantages. It is
the responsibility of manufacturing management to select the best combination of these
basic plans so total costs are minimized and customer service levels are maintained.
A make-to-stock production plan determines how much to produce in each period to
meet the following objectives:
■■
■■
Achieve the forecast.
Maintain the required inventory levels.
Although demand must be satisfied, the plan must balance the costs of maintaining
inventory with the cost of changing production levels.
A make-to-order production plan determines how much to produce in each period to
meet the following objectives:
■■
■■
Achieve the forecast.
Maintain the planned backlog.
The cost of a backlog that is too large equals the cost of turning away business. If customers have to wait too long for delivery, they might take their business elsewhere. As with
a make-to-stock production plan, demand must be satisfied, and the plan must balance the
costs of changing production levels with the cost of a backlog that is larger than desired.
Production Planning System
37
key termS
Advanced Planning and Scheduling (APS)
systems 27
Aggregate production plan 22
Backlog 34
Backorder 32
Balanced scorecard 21
Business plan or strategic business plan 21
Capacity 18
Chase strategy 29
Enterprise resource planning (ERP) 27
Green production 25
Hoshin planning 20
Hybrid strategy 31
Key Performance Indicators (KPI) 21
Level production plan 32
Manufacturing resource plan (MRP II) 26
Master production schedule (MPS) 22
Material requirements plan (MRP) 22
Priority 18
Production activity control (PAC) 23
Production leveling 30
Production plan 22
Resource bill 35
Resource planning 35
Remanufacturing 21
Reverse logistics 21
Reverse supply chain 21
Risk management 21
Sales and operations planning (SOP) 24
Social responsibility 20
Strategic plan 20
Subcontracting 30
Sustainability 20
United Nations Global Compact 20
QueStionS
1. What are the four questions a good planning system must answer?
2. Define capacity and priority. Why are they important in production planning?
3. Describe each of the following plans in terms of their purpose, planning horizon, level of detail,
and planning cycle:
a. Strategic plan.
b. Business plan.
c. Production plan.
d. Master production schedule.
e. Material requirements plan.
f. Production activity control.
4. Describe the responsibilities and inputs of the marketing, production, finance, and engineering
departments to the strategic business plan.
5. Describe the relationship among the production plan, the master production schedule, and the
material requirements plan.
6. What is the difference between strategic business planning and sales and operations planning
(SOP)? What are the major benefits of SOP?
7. What is MRP II?
8. What is ERP?
9. What two changing conditions led to the development of ERP systems?
10. In the short run, how can capacity be changed?
11. When making a production plan, why is it necessary to select a common unit or to establish
product families?
12. On what basis should product groups (families) be established?
13. What are five typical characteristics of the production planning problem?
14. Describe each of the four basic strategies used in developing a production plan. What are the
advantages and disadvantages of each?
15. What is a hybrid strategy? Why is it used?
38
Chapter two
16. Describe four conditions under which a firm would make-to-stock or make-to-order.
17. What information is needed to develop a make-to-stock production plan?
18. What are the steps in developing a make-to-stock production plan?
19. What is the difference between make-to-order and assemble-to-order? Give an example of each.
20. What information is needed to develop a make-to-order production plan? How does this differ
from that needed for a make-to-stock plan?
21. What is the general procedure for developing a level production plan in a make-to-order environment?
22. What is a resource bill? At what level in the planning hierarchy is it used?
23. What kind of production environment would you expect to see if a company uses a chase strategy? What if it uses a level strategy?
24. What does the concept of “green production” mean? How will it potentially impact production
planning?
25. Describe sustainability for production. What are some of the ways that a company can practice
sustainability?
26. What should a company approach be to risks? What are some of the methods they can use to
minimize the impact of negative risks?
27. Describe some of the possible advantages of a company developing a reverse supply chain.
28. What is Hoshin planning?
ProblemS
2.1. If the opening inventory is 400 units, demand is 900 units, and production is 700 units,
what will be the ending inventory?
Answer. 200 units
2.2. A company wants to produce 480 units over the next three months at a uniform rate.
The months have 19, 20, and 21 working days, respectively. On the average, how
much should the company produce each day to level production?
Answer. Average daily production = 8 units
2.3. A company plans to produce 25,000 units in a 3-month period. The months have 22,
21, and 20 working days, respectively. What should the average daily production be?
2.4. In problem 2.2, how much will be produced in each of the three months?
Answer. Month 1: 158 Month 2: 166 Month 3: 174
2.5. In problem 2.3, how much will be produced in each of the three months?
2.6. A production line is to run at 1000 units per month. Sales are forecast as shown in
the following. Calculate the expected period-end inventory. The opening inventory is
600 units. All periods have the same number of working days.
Period
1
2
3
4
5
6
Forecast
750
800
1050
1600
1000
850
Planned Production
1000
1000
1000
1000
1000
1000
Planned
Inventory
600
Answer. For period 1, the ending inventory is 850 units.
2.7. A company wants to develop a level production plan for a family of products. The
opening inventory is 100 units, and an increase to 150 units is expected by the end of
Production Planning System
39
the plan. The demand for each period is given in what follows. How much should the
company produce each period? What will be the ending inventories in each period?
All periods have the same number of working days.
Period
1
2
3
4
5
6
Forecast Demand
100
120
125
130
115
110
Total
Planned Production
Planned
Inventory
Answer.
100
Total production = 750 units.
Period production = 125 units .
The ending inventory for period 1 is 125; for period 5, 135.
2.8. A company wants to develop a level production plan for a family of products. The opening inventory is 550 units, and a decrease to 200 units is expected by the end of the plan.
The demand for each of the periods is given in what follows. All periods have the same
number of working days. How much should the company produce each period? What
will be the ending inventories in each period? Do you see any problems with the plan?
Period
Forecast Demand
1
2
3
4
5
6
1300
1200
800
600
800
900
Total
Planned Production
Planned
Inventory
550
2.9. A company wants to develop a level production plan. The beginning inventory is
zero. Demand for the next four periods is given in what follows.
a. What production rate per period will give a zero inventory at the end of period 4?
b. When and in what quantities will backorders occur?
c. What level production rate per period will avoid backorders? What will be the ending inventory in period 4?
Period
1
2
3
4
Forecast Demand
9
5
9
9
Planned Production
Planned Inventory
Answer.
0
a. 8 units
b. period 1, minus 1
c. 9 units; 4 units
Total
40
Chapter two
2.10. If the cost of carrying inventory is $60 per unit per period and stockout cost $500 per
unit, what will be the cost of the plan developed in problem 2.9a? What will be the
cost of the plan developed in 2.9c?
Answer.
Total cost for plan in question 2.9a = $650
Total cost for plan in question 2.9c = $600
2.11. A company wants to develop a level production plan for a family of products. The
opening inventory is 100 units, and an increase to 130 units is expected by the end
of the plan. The demand for each month is given in what follows. Calculate the total
production, daily production, and production and ending inventory for each month.
Month
May
Jun
Jul
Aug
Working Days
21
19
20
10
Forecast Demand
115
125
140
150
Total
Planned Production
Planned Inventory
Answer.
100
Total monthly production for May = 168 units
The ending inventory for May = 153 units
2.12. A company wants to develop a level production plan for a family of products. The
opening inventory is 500 units, and a decrease to 250 units is expected by the end of
the plan. The demand for each of the months is given in what follows. How much
should the company produce each month? What will be the ending inventory in each
month? Do you see any problems with the plan?
Month
Jan
Fe b
Mar
Apr
May
Jun
Working Days
20
22
20
20
18
19
Forecast Demand
1200
1300
800
700
700
900
Total
Planned Production
Planned
Inventory
500
2.13. Because of its labor contract, a company must hire enough labor for 100 units of
production per week on one shift or 200 units per week on two shifts. It cannot hire,
lay off, or assign overtime. During the fourth week, workers will be available from
another department to work part or all of an extra shift (up to 100 units). There is a
planned shutdown for maintenance in the second week, which will cut production to
half. Develop a production plan. The opening inventory is 200 units, and the desired
ending inventory is 300 units.
Week
1
2
3
4
5
6
Forecast Demand
120
160
240
240
160
160
Planned Production
Planned
Inventory
Total
Production Planning System
41
2.14. If the opening backlog is 450 units, forecast demand is 700 units, and production is
800 units, what will be the ending backlog?
Answer. 350 units
2.15. The opening backlog is 800 units. Forecast demand is as shown here. Calculate the
weekly production for level production if the backlog is to be reduced to 100 units.
Week
1
2
3
4
5
6
Forecast Demand
750
700
550
700
600
500
Total
Planned Production
Projected
Backlog
800
Answer.
Total production = 4500 units
Weekly production = 750 units
Backlog at end of week 1 = 800 units
2.16. The opening backlog is 1100 units. Forecast demand is as shown here. Calculate
the weekly production for level production if the backlog is to be increased to
1200 units.
Week
Forecast Demand
1
2
3
4
5
6
1200
1100
1200
1200
1100
1000
Total
Planned Production
Projected
Backlog
1100
2.17. For the following data, calculate the number of workers required for level production and the resulting month-end inventories. Each worker can produce 15 units per
day, and the desired ending inventory is 9000 units.
Month
1
2
3
4
Working Days
20
24
12
19
Forecast Demand
28,000
27,500
28,500
28,500
Planned Production
Planned Inventory
Answer.
11,250
Workers needed = 98 workers
First month’s ending inventory = 12,650 units
Total
42
Chapter two
2.18. For the following data, calculate the number of workers required for level production and the resulting month-end inventories. Each worker can produce 9 units per
day, and the desired ending inventory is 800 units. Why is it not possible to reach the
exact ending inventory target?
Month
1
2
3
4
5
6
Working Days
20
24
12
22
20
19
Forecast Demand
2800
3000
2700
3300
2900
3200
Total
Planned Production
Planned
Inventory
1000
caSe Study 2.1
Meridian Water Pumps
John Lopez, president of Meridian Water Pumps (a small water pump producer), was
holding a meeting with his department managers. They were in the process of planning
production of medium-sized pumps for the next six months. Mr. Lopez tolerated some of
the arguments before he felt it necessary to stop the discussion as it was going so that he
could direct it toward a solution. A summary of some of the arguments follows:
Mary Welch, marketing and sales manager: “My sales people are very good, but get
very frustrated at times. Several times last year the sales people spent lots of their time trying to calm down frustrated customers. As they are supposed to do, the sales people sold
as many of these pumps as they could, yet at times the production could not keep up with
the orders. Production knows that we have some cyclicality in the demand, but we have
plenty of machine capacity. They should be able to hire people so that we can meet the
demand that we sell. Why can’t they get their area to work correctly?”
Frank Jackson, production manager: “Come on, Mary, we know the sales are cyclical,
but we never know exactly when the cycles happen. Even if we did, the Human Resource
(HR) people always take too long to get us the people we need. By the time we get the
new people hired and trained, the sales seem to drop again. What am I supposed to do?
If we keep them and allow them to keep producing pumps, our inventory climbs and the
finance people start yelling. I can’t just let those new people sit around doing nothing. The
only other alternative is to lay them off, but then the HR people get really angry.”
Elizabeth Conrad, human resource manager: “You bet we get angry. The production people will occasionally start pushing us to hurry and hire more people, yet get very
impatient. It takes time to go through the interview process and get people hired and oriented
to our business. Then we no sooner get them on site and working when production asks us to
lay them off. That is a real problem for two reasons. First, there are costs involved. It takes
an average of $100 to get a person hired, and another $100 to lay them off. Second, those
people that we hire and then quickly lay off tend to not return. I can’t blame them, since
from their perspective it looks like we have no idea how to run our business. In addition, as
those people complain to other people about our treatment of them, our reputation is getting
to look bad, and that makes it increasingly difficult to find good people to hire.”
Joseph Western, finance manager: “Frank is correct that I get upset when the inventory climbs. It costs us about $5 to keep one of these pumps in our inventory for a month.
That cost comes right out of our profit. Since my job is to maximize profitability, I can’t
sit by and let those inventory dollars shrink that profitability. The same goes for all that
hiring and layoffs. That money also hurts profitability. Can’t we do better?”
Production Planning System
43
At this point Mr. Lopez stopped the discussion and said “Enough of trying to blame
each other. It is our job as managers to manage this process more effectively. Marketing
has just completed a 6-month forecast of anticipated demand for this family of pumps, and
we know from past history that their forecasts are pretty good. We should be able to come
up with an approach that we all can live with and focus our efforts to meet. Let’s get to
work on it.”
assignment
Assume you have been given the job to develop an effective approach to the problem.
First, here is the forecast developed by marketing:
Month
1
2
3
4
5
6
Forecasted Demand
600
750
1000
850
750
700
The production manager said there were currently 50 units in inventory, and they
would like to end the six months with only 25 in inventory. He also said that currently
each worker produces an average of 25 pumps in any given month. There are currently
20 workers in the medium-size pump area.
1. Using the data, develop a level production plan. How much extra cost (inventory and
HR costs) are involved in this plan? What additional costs (both financial and nonfinancial) might be involved with such a plan?
2. Using the data, develop a chase production plan. How much extra cost (inventory and
HR costs) are involved in this plan? What additional costs (both financial and nonfinancial) might be involved with such a plan?
3. Try to develop a possible hybrid plan that would accomplish the task with smaller
total costs than either level or chase.
4. Based on your work, what would you recommend and why? What are some of the
pros and cons of the solution you recommend?
caSe Study 2.2
Williams 3D Printers
The Williams 3D printer company was experiencing growing pains early in 2015. Jasper
Williams had developed his own unique design for making a 3D printer with this relatively new technology that was growing fairly rapidly in interest and in competitors. He
had started by using his engineering skills as an individual inventor, but with some borrowed money he was able to set up a small production facility. His sales the first year
were modest, as he made and sold only five printers. Now that he had been in the business
for three years, he noticed that near the end of his fiscal year, he was likely to sell more
than 20 units. He only had three other people on his management staff: John Johnson, the
financial officer, Pamela Lopez, the production manager, and Mary Andrews, the marketing and sales manager. The following conversation took place during their most recent
monthly planning meeting, where the key item on the agenda was to look at plans for the
next fiscal year:
jasper: “Mary, I think you mentioned that we are gaining a good reputation in a market that is growing rapidly, given that it is in the early stages of the life cycle.
What do you think that means for sales this coming year?”
mary: “I think our good reputation is going to be a real plus. Not only do several of
our original customers plan on buying another printer from us, but they have
also told other potential customers that they like our design, and some of those
potential customers are likely to buy as well. I think it is very likely that we
could double our sales next year to possibly 40 or more printers.”
44
Chapter two
pamela: “We need to talk about that—perhaps you should hold back on making
sales like that. We are already finding it tough to deliver on promises for this
year. We have had a couple of late orders this year, and the only reason we
didn’t have more late orders was that our workers agreed to work on some
weekends. Problem is, I don’t know how agreeable they will be to that next
year. While they like the extra money, they all have families and don’t want to
spend that much time away from home.”
mary: “Look, Pamela, we have worked hard to get the good reputation to increase
our sales. What good does it do if we can’t meet the needs of people who want
to buy from us? Last I knew, we were in business to make sales and therefore
make money. We have a good profit margin on the printers that should make the
company very profitable.”
pamela: “Well the only way we can really expand to sell and make 40 units next
year is to hire a lot more people. We possibly could hire a second shift, but our
workers are skilled people who are making most of these printers in a somewhat
unique design based on the specific needs of the customer. Skilled people like
that often have several options as to where to work, and I am pretty sure most
would not like to work a second shift where they could not spend evenings with
their families. We could try to double the number of workers on the regular day
shift, but that would mean duplicating all the current equipment as well. Also,
while today we probably have the space in our facility to fit duplicate equipment, if we grow more the following year we will also have to expand our space
requirements.”
At this point John Johnson, the financial manager, had to break into the
conversation:
john: “We need to think long and hard about all this. After three years we are finally looking a little better financially, but adding a whole lot more people
and equipment is going to cost us a lot. Also, you have to keep in mind that
the printers have a pretty long lead time to produce, given that each customer
specifies at least some unique aspect of design based on their individual needs,
and also it takes a fair amount of time to build them. That means we get to see
the money from the sale only after many weeks after the order is placed, but
in the meantime we have to obtain materials and pay workers for today’s new
orders—which are larger in number in this growing market we have. This implies that even though the profit per unit is good, we have a struggle with cash
flow. Pamela, what about adding just one or two people as the sales grow, and
then add some more later in the year as the sales continue to grow?”
pamela: “I don’t see how that can help. Adding a duplicate person without additional equipment for them to work with makes no sense, and even if they could
do the work, the other people in areas we don’t add would still have a lot more
work to do without the time to do it.”
john: “Well we might be able to get another loan to help, but do we really want to do
that just at the point we are starting to show some bottom-line profit?”
jasper: “Okay, I understand each of your perspectives, but arguing back and forth
doesn’t help. We need to figure out something to do that we can all agree on.
Let’s get to work.”
assignment
Try to help the management team out. After listing the key issues and characteristics of
the environment and the problem, list all the possible alternative approaches they could
take to deal with the issues. For each alternative, try to list the pros and cons likely involved. Then select an approach you would recommend and attempt to justify it.
Chapter
three
Master scheduling
introduction
After production planning, the next step in the manufacturing planning and control process is to prepare a master production schedule (MPS). This chapter examines some
basic considerations in making and managing an MPS. It is an extremely important planning tool and forms the basis for communication between sales and manufacturing. The
MPS is a vital link in the production planning system.
It forms the link between production planning and what manufacturing will actually
build. From this perspective it forms the major link between customer demand and the
production facility.
■■ It forms the basis for calculating the capacity and resources needed.
■■ It drives the material requirements plan. As a schedule of items to be built, the MPS
and bills of material determine what components are needed from manufacturing and
purchasing.
■■ It keeps priorities valid. The MPS is a priority plan for manufacturing.
■■
Whereas the production plan deals in families of products, the MPS works with
end items. It breaks down the production plan into the requirements for individual end
items, in each family, by date and quantity. The production plan limits the MPS, that
is, the total of the items in the MPS should not be different from the total shown on the
production plan. For example, if the production plan shows a planned production of 1000
tricycles in a particular week, the total of the individual models planned for by the MPS
should be 1000. Within this limit, its objective is to balance the demand (priorities) set
by the marketplace with the availability of materials, labor, and equipment (capacity) of
manufacturing.
The end items made by the company are assembled from component and subcomponent parts. These must be available in the right quantities at the right time to support
the master production schedule. The material requirements planning system plans the
schedule for these components based on the needs of the MPS. Thus, the MPS drives the
material requirements plan.
The MPS is a plan for manufacturing. It reflects the needs of the marketplace and the
capacity of manufacturing and forms a priority plan for manufacturing to follow.
The MPS forms a vital link between sales and production:
It makes possible valid order promises. The MPS is a plan of what is to be produced
and when. As such, it tells sales and manufacturing when goods will be available for
delivery.
■■ It is an agreed-upon plan, and a contract between marketing and manufacturing.
■■
The MPS forms a basis for sales and production to determine what is to be manufactured. It is not meant to be rigid. It is a device for communication and a basis to make changes
that are consistent with the demands of the marketplace and the capacity of manufacturing.
The information needed to develop an MPS is provided by:
The production plan—the aggregated production plan developed during the S&OP
process.
■■ Forecasts for individual end items.
■■ Actual orders received from customers and for stock replenishment.
■■
45
46
Chapter three
■■
■■
Inventory levels for individual end items.
Capacity restraints.
relationship to production plan
Suppose the following production plan is developed for a family of three items:
Week
1
2
3
4
5
6
Aggregate Forecast (units)
160
160
160
160
215
250
Production Plan
205
205
205
205
205
205
Aggregate Inventory (units)
545
590
635
680
670
625
Opening inventories (units) are
Product A
350
Product B
100
Product C
50
Total
500
The next step is to forecast demand for each item in the product family. Keep in
mind that the forecast data for the production plan was based on aggregated data (product
families), while for the master schedule, the more specific forecast is needed for each item
in the item family. Forecasts for the sales and operations planning process used product
families since forecasts are often more accurate for product families than for individual
products. Product family forecasts were all that was needed for the S&OP since it was
being used to plan resources needed to produce the product family forecast and not individual item production. It should also be noted that while individual item forecasts might
not add up to the exact product family forecast, that production plan for the product family
represents a constraint on the total that can be produced for items in the family.
Week
1
2
3
4
5
6
Product A
70
70
70
70
70
80
Product B
40
40
40
40
95
120
Product C
50
50
50
50
50
50
Total
160
160
160
160
215
250
With this data, the master scheduler must now devise a plan to fit the constraints. The
following illustrates a possible solution.
Master Schedule
Week
1
2
3
4
5
Product A
Product B
205
205
205
205
Product C
Total Planned
6
205
205
205
205
205
205
205
205
Master Scheduling
47
Inventory
Week
1
2
3
4
5
6
Product A
280
210
140
70
0
125
Product B
265
430
595
555
460
340
Product C
0
–50
–100
55
210
160
Total Planned
545
640
735
680
670
625
This schedule is satisfactory for the following reasons:
■■
■■
It tells the plant when to start and stop production of individual items.
Capacity is consistent with the production plan.
It is unsatisfactory for the following reasons:
It has a poor inventory balance compared to total inventory.
■■ It results in a stockout for product C in periods 2 and 3.
■■
The term master production schedule refers to the last line of the matrix. The term
master scheduling refers to the process of arriving at that line. Thus, the total matrix is
called a master schedule.
Example Problem
The Hotshot Lightning Rod Company makes a family of two lightning rods, Models H
and I. It bases its production planning on weeks. For the present month, production is
leveled at 1000 units. Opening inventory is 500 units, and the plan is to reduce that
to 300 units by the end of the month. The MPS is made using weekly periods. There
are 4 weeks in this month, and production is to be leveled at 250 units per week. The
forecast and projected available for the two lightning rods follows. Calculate an MPS for
each item.
Answer
Production Plan
Week
1
2
3
4
Total
Forecast
300
350
300
250
1200
450
350
300
300
250
250
250
250
1000
Week
1
2
3
4
Total
Forecast
200
300
100
100
700
250
200
100
100
250
250
Projected Available
500
Production Plan
Master Schedule: Model H
Projected Available
MPS
200
100
48
Chapter three
Master Schedule: Model I
Week
1
2
3
4
Total
Forecast
100
50
200
150
500
200
150
200
200
250
150
Projected Available
300
MPS
developing a Master production schedule
The objectives in developing an MPS are as follows:
To maintain the desired level of customer service by maintaining finished-goods
inventory levels or by scheduling to meet customer delivery requirements.
■■ To make the best use of material, labor, and equipment.
■■ To maintain inventory investment at the required levels.
■■
To reach these objectives, the plan must satisfy customer demand, be within the
capacity of manufacturing, and be within the guidelines of the production plan.
There are three steps in preparing an MPS:
1. Develop a preliminary MPS.
2. Check the preliminary MPS against available capacity.
3. Resolve differences between the preliminary MPS and capacity availability.
Preliminary Master Production Schedule
To show the process of developing an MPS, an example is used that assumes that the
product is made to stock, an inventory is kept, and the product is made in lots.
A particular item is made in lots of 100, and the expected opening inventory is 80
units. Figure 3.1 shows the forecast of demand, the projected available on hand, and the
preliminary MPS.
Period 1 begins with an inventory of 80 units. After the forecast demand for 60 units
is satisfied, the projected available is 20 units. A further forecast demand of 60 in period
2 is not satisfied, and it is necessary to schedule an MPS receipt of 100 for week 2. This
produces a projected available of 60 units 120 + 100 - 60 = 602 at the end of period 2.
In period 3, the forecast demand for 60 is satisfied by the projected 60 on hand, leaving a
projected available of 0. In period 4, a further 100 must be received, and when the forecast
demand of 60 units is satisfied, 40 units remain in inventory.
This process of building an MPS occurs for each item in the family. If the total
planned production of all the items in the family and the total ending inventory do not
On hand = 80 units
Lot size = 100 units
Period
1
2
3
4
5
6
Forecast
60
60
60
60
60
60
20
60
0
40
80
20
100
100
Projected Available
MPS
FigurE 3.1 MPS example.
80
100
Master Scheduling
49
agree with the production plan, some adjustment to the individual plans must be made so
the total production is the same.
Once the preliminary master production schedules are made, they must be checked
against the available capacity. This process is called rough-cut capacity planning.
Example Problem
Amalgamated Nut Crackers, Inc., makes a family of nut crackers. The most popular
model is the walnut, and the sales department has prepared a 6-week forecast. The
opening inventory is 50 dozen (dozen is the unit used for planning). As master planner, you must prepare an MPS. The nutcrackers are made in lots of 100 dozen.
Answer
Week
1
2
3
4
5
6
Forecast Sales
75
50
30
40
70
20
75
25
95
55
85
65
Projected Available
50
MPS
100
100
100
Rough-Cut Capacity Planning
Rough-cut capacity planning checks whether critical resources are available to support the
preliminary master production schedules. Critical resources include bottleneck operations,
labor, and critical materials (perhaps material that is scarce or has a long lead time).
The process is similar to resource requirements planning used in the production planning process. The difference is that now we are working with a product and not a family
of products. The resource bill, used in resource requirements planning, assumes a typical
product in the family. Here the resource bill is for a single product. As before, the only
interest is in bottleneck work centers and critical resources.
One reason this method is described as “rough” is not only because it focuses primarily on critical resources but also because there are several other variables that can impact
specific details of capacity usage. These include existing inventory, existing work orders
that are partially complete, and lead times. Inclusion of those and other capacity variables
are discussed in more detail in Chapter 5.
Suppose a firm manufactures four models of desktop computers assembled in a work
center that is a bottleneck operation. The company wants to schedule to the capacity of
this work center and not beyond. Figure 3.2 shows a resource bill for that work center
showing the time required to assemble one computer.
Resource Bill
Desktop Computer Assembly
Computer
Assembly Time
(standard hours)
Model D24
Model D25
Model D26
Model D27
0.203
0.300
0.350
0.425
FigurE 3.2 Resource bill.
50
Chapter three
Suppose that in a particular week the master production schedules show that the following computers are to be built:
Model D24
Model D25
Model D26
Model D27
200 units
250 units
400 units
100 units
The capacity required on this critical resource is
Model D24 200 * 0.203 = 40.6 standard hours
Model D25 250 * 0.300 = 75.0 standard hours
Model D26 400 * 0.350 = 140.0 standard hours
Model D27 100 * 0.425 = 42.5 standard hours
Total time required = 298.1 standard hours
Example Problem
The Acme Tweezers Company makes tweezers in two models, medium and fine. The
bottleneck operation is in work center 20. Following is the resource bill (in hours per
dozen) for work center 20.
Hours per Dozen
Work Center
Medium
Fine
20
0.5
1.2
The master production schedule for the next 4 weeks is
Week
1
2
3
4
Total
Medium
40
25
40
15
120
Fine
20
10
30
20
80
Using the resource bill and the master production schedule, calculate the number of hours required in work center 20 for each of the 4 weeks. Use the following
table to record the required capacity on the work center.
Answer
Week
1
2
3
4
Total
Medium
20
12.5
20
7.5
60
Fine
24
12
36
24
96
Total Hours
44
24.5
56
31.5
156
Master Scheduling
51
Resolution of Differences
The next step is to compare the total time required to the available capacity of the work
center. If available capacity is greater than the required capacity, the MPS is workable.
If not, methods of increasing capacity have to be investigated. Is it possible to adjust the
available capacity with overtime, extra workers, routing through other work centers, or
subcontracting? If not, it will be necessary to revise the master production schedule.
Finally, the master production schedule must be judged by three criteria:
1. Resource use. Is the MPS within capacity restraints in each period of the plan? Does
it make the best use of resources?
2. Customer service. Will due dates be met and will delivery performance be acceptable?
3. Cost. Is the plan economical, or will excess costs be incurred for overtime, subcontracting, expediting, or transportation?
Master Schedule Decisions
The MPS should represent as efficiently as possible what manufacturing will make. If too
many items are included, it will lead to difficulties in forecasting and managing the MPS.
In each of the manufacturing environments—make-to-stock, make-to-order, assemble-toorder, configure-to-order, and engineer-to-order—master scheduling should take place
where the smallest number of product options exists. Figure 3.3 shows the level at which
items should be master scheduled.
Make-to-stock products In this environment, a limited number of standard items are
assembled from many components. Televisions and other consumer products are examples. The MPS is usually a schedule of finished goods items.
Make-to-order products In this environment, many different end items are made
from a small number of components. Custom-tailored clothes are an example. The MPS is
usually a schedule of raw material replenishment.
Assemble-to-order and configure-to-order products In these environments,
many end items can be made from combinations of basic components, subassemblies, features,
and options. For example, suppose a company manufactures paint from a base color and adds
tints to arrive at the final color. Suppose there are 10 tints and a final color is made by mixing any three of them with the base. There are 720 possible colors 110 * 9 * 8 = 7202.
Forecasting and planning production for 720 items is a difficult task. It is much easier if
production is planned at the level of the base color and the 10 tints. There are then only 10 items
with which to deal: the base color and each of the 10 tints. Once a customer’s order is received,
the base color and the required tints can be combined (assembled) according to the order.
Final assembly schedule (FAS) This step, assembly to customer order, is generally
planned using a final assembly schedule. This is a schedule of what will be assembled. It
Make to
Stock
Make to
Order
Assemble to
Order
End
Product
End
Product
End
Product
MPS
FAS
FAS
MPS
MPS
Raw
Material
Raw
Material
FigurE 3.3 Different MPS environments.
Raw
Material
52
Chapter three
ORDER ENTRY
AND
PROMISE
PRODUCTION
PLAN
RESOURCE
REQUIREMENTS
PLAN
LONG RANGE
FINAL
ASSEMBLY
SCHEDULE
MASTER
PRODUCTION
SCHEDULE
ROUGH CUT
CAPACITY
PLAN
MEDIUM RANGE
MATERIAL
REQUIREMENTS
PLAN
CAPACITY
REQUIREMENTS
PLAN
SHORT RANGE
PURCHASING
AND
PRODUCTION
ACTIVITY
CONTROL
INPUT/OUTPUT
CONTROL
AND
OPERATION
SEQUENCING
IMMEDIATE
RANGE
FigurE 3.4 MPS, FAS, and other planning activities.
is used when there are many options and it is difficult to forecast which combination the
customers will want. Master production scheduling is done at the component level, for
example, the base color and tint level. The final assembly takes place only when a customer order is received.
The FAS schedules customer orders as they are received and is based on the components planned in the MPS. It is responsible for scheduling from the MPS through final
assembly and shipment to the customer. The FAS is typically used in both the ATO and
MTO environments.
Engineer-to-order This is a form of make-to-order (MTO) products. In this environment, the product is designed before manufacturing, based on the customer’s very special
needs. A bridge is an example.
Figure 3.4 shows the relationship of the MPS, the FAS, and other planning
activities.
It should be noted that the master schedule is typically designed to plan production
as “close” as possible to the customer. Make-to-stock products, for example, are master
scheduled as the final product design to provide rapid response to customer demand.
An additional advantage is that the production of the product itself is “insulated” from
customer influence, implying more internal stability. Make-to-order products, on the
other hand, have significant customer input on the final design. The master schedule for
make-to-order products is often at the raw material stage, meaning customer influence and
possible associated production disruptions are high. The assemble-to-order products are
somewhere in-between. The master schedule is often for modules or options (and common parts for all products), while the exact customer-specified design is reflected in the
final assembly schedule. The customer tends to have little influence on the specific design
of the option, thereby insulating the actual production of that option. For example, you
as a customer may buy a particular model of automobile. While you may have influence
on whether the engine is a 6- or 8-cylinder engine, you have no influence on how either
of those engines is produced. Some companies actively design products to shift customer
influence on the final design as close as possible to the furthest point in the supply chain.
This design strategy is called postponement.
Master Scheduling
B
(LT = 6)
A
(LT = 1)
C
(LT = 5)
E
D
(LT = 2)
(LT = 2)
F
(LT = 3)
G
(LT = 6)
53
FigurE 3.5 Product structure: critical lead time.
Planning Horizon
The planning horizon is the time span for which plans are made. It must cover a period at
least equal to the time required to accomplish the plan. For master production scheduling,
the minimum planning horizon is the longest cumulative or end-to-end lead time (LT).
For example, in Figure 3.5, the longest cumulative lead time path is A to D to F to G.
The cumulative lead time is 1 + 2 + 3 + 6 = 12 weeks. The minimum planning horizon must be 12 weeks; otherwise, raw material G would not be ordered in time to meet
delivery.
The planning horizon is usually longer for several reasons. The longer the horizon,
the greater the visibility and the better management’s ability to avoid future problems or
to take advantage of special circumstances. For example, firms might take advantage of
economical purchase plans, avoid future capacity problems, or manufacture in more economical lot sizes.
As a minimum, the planning horizon for a final assembly schedule must include time
to assemble a customer’s order. It does not need to include the time necessary to manufacture the components. That time will be included in the planning horizon of the MPS.
production planning, Master scheduling,
and sales
The production plan reconciles total forecast demand with available resources. It takes
information from the strategic plan, the strategic business plan, and market forecasts to
produce an overall plan of what production intends to make to meet forecast. It is a major
output of the sales and operations planning process discussed in Chapter 2. It is dependent
on the forecast and, within capacity limits, must plan to satisfy the forecast demand. It is
not concerned with the detail of what will actually be made. It is intended to provide a
framework in which detailed plans can be made in the MPS.
The MPS is built from forecasts and actual demands for individual end items. It reconciles demand with the production plan and with available resources to produce a plan
that manufacturing can fulfill. The MPS is concerned with what items will actually be
built, in what quantities, and when, to meet expected demand.
The production plan and the MPS uncouple the sales forecast from manufacturing by
establishing a manufacturing plan. Together, they attempt to balance available resources
of plant, equipment, labor, and material with forecast demand. However, they are not a
sales forecast, nor are they necessarily what is desired. The MPS is a plan for what production can and will do.
Figure 3.6 shows the relationship among the sales forecast, production plan, and
MPS.
Chapter three
SALES
FORECAST
PRODUCTION
PLAN
MASTER
PRODUCTION
SCHEDULE
MATERIAL
REQUIREMENTS
PLAN
FigurE 3.6 Sales forecast, production plan, and master production schedule.
The MPS must be realistic about what manufacturing can and will do. If it is not, it
will result in overloaded capacity plans, past-due schedules, unreliable delivery promises,
surges in shipments, and lack of accountability.
The MPS is a plan for specific end items or “buildable” components that manufacturing expects to make over some time in the future. It is the point at which manufacturing
and marketing must agree what end items are going to be produced. Manufacturing is
committed to making the goods, and marketing to selling the goods. However, the MPS
is not meant to be rigid. Demand changes, problems occur in production, and, sometimes,
components are scarce. These events may make it necessary to alter the MPS. Changes
must be made with the full understanding and agreement of sales and production. The
MPS provides the basis for making changes and a plan on which all can agree.
The MPS and Delivery Promises
In a make-to-stock environment, customer orders are satisfied from inventory. However,
in make-to-order or assemble-to-order environments, demand is satisfied from production
capacity. In either case, sales and distribution need to know what is available to satisfy
customer demand. Since demand can be satisfied either from inventory or from scheduled
receipts, the MPS provides a plan for doing either. Figure 3.7 illustrates the concept. As
orders are received, they “consume” the available inventory or capacity. Any part of the
plan that is not consumed by actual customer orders is available to promise to customers.
In this way, the MPS provides a realistic basis for making delivery promises. It should be
noted that for each period, the MPS quantity is assumed to be the quantity available at the
beginning of that period.
Using the MPS, sales and distribution can determine the available to promise (ATP).
Available to promise is that portion of a firm’s inventory and planned production that is
not already committed and is available to the customer. This allows delivery promises to
be made and customer orders and deliveries to be scheduled accurately.
The ATP is calculated by adding scheduled receipts to the beginning inventory and
then subtracting actual orders scheduled before the next scheduled receipt. A scheduled
CAPACITY
UNITS
54
BOOKED
ORDERS
AVAILABLE
TO PROMISE
TIME
FigurE 3.7 The MPS and delivery time.
Master Scheduling
55
Inventory on hand: 100 units
Period
1
2
3
Customer Orders
80
10
10
MPS
ATP
20
4
5
30
100
100
80
70
FigurE 3.8 Available to promise calculation.
receipt is an order that has been issued either to manufacturing or to a supplier. Figure 3.8
illustrates a calculation of an ATP:
ATP for period 1 = on hand - customer orders due before next MPS
= 100 - 80
= 20 units
ATP for period 2 = MPS scheduled receipt - customer orders due before next MPS
= 100 - 110 + 102
= 80 units
ATP for period 4 = 100 - 30 = 70 units
This method assumes that the ATP will be sold before the next scheduled receipt
arrives. It is there to be sold, and the assumption is that it will be sold. If it is not sold,
whatever is left forms an on-hand balance available for the next period.
Continuing with the example problem on page 49, Amalgamated Nut Crackers, Inc.,
has now received customer orders. Following is the schedule of orders received and the
resulting available to promise calculation (recall that there are 50 units in inventory):
Inventory on hand = 50 units
Week
1
2
3
4
5
6
Customer Orders
80
45
40
50
50
5
MPS
100
100
100
ATP
25
10
45
Sometimes, customer orders are greater than the scheduled receipts. In this case, the
previous ATP is reduced by the amount needed. In this example, can the master planner
accept an order for another 20 for delivery in week 3? Ten of the units are available from
week 3, and 10 can be taken from the ATP in week 1, so the order can be accepted as
shown in the following.
Inventory on hand = 50 units
Week
1
2
3
4
5
6
Customer Orders
80
45
60
50
50
5
MPS
100
100
100
ATP
15
0
45
56
Chapter three
Example Problem
Calculate the available to promise for the following example. Can an order for 30 more
be accepted for delivery in week 5? What will be the ATP if the order is accepted?
Week
1
2
3
4
5
Customer Orders
50
20
30
30
15
MPS
100
100
ATP
30
25
Answer
Week
1
2
3
4
5
Customer Orders
50
20
30
30
45
MPS
100
100
ATP
25
0
Projected Available Balance
Our calculations so far have based the projected available balance on the forecast demand.
Now there are also customer orders to consider. Customer orders will sometimes be
greater than forecast and sometimes less. Projected available balance is now calculated
based on whichever is greater. For example, if the beginning projected available balance is
100 units, the forecast is 40 units, and customer orders are 50 units, the ending projected
available balance is 50 units, not 60. The projected available balance (PAB) is calculated in one of two ways, depending on whether the period is before or after the demand
time fence. The demand time fence is the number of periods, beginning with period 1, in
which changes are not accepted due to excessive cost caused by schedule disruption.
For periods before the demand time fence, PAB is calculated as
PAB = prior period PAB or on@hand balance + MPS - customer orders
This process ignores the forecast and assumes that the only effect will be from the customer orders. Any new orders will have to be approved by senior management. For periods
after the demand time fence, forecast will influence the PAB so it is calculated using either
the forecast or customer orders, whichever is greater. Thus, the PAB becomes
PAB = prior period PAB or on@hand balance + MPS
- greater of customer orders or forecast
Example Problem
Given the following data, calculate the projected available balance. The demand time
fence is the end of week 3, the order quantity is 100, and 40 are available at the
beginning of the period.
Week
1
2
3
4
5
Forecast
40
40
40
40
40
Customer Orders
39
42
39
33
23
Master Scheduling
57
Answer
Week
Projected Available Balance
40
1
2
3
4
5
1
59
20
80
40
MPS
100
100
So far we have considered how to calculate projected available balance and available
to promise. Using the Amalgamated Nut Cracker, Inc., example, we now combine the two
calculations into one record. The demand time fence is at the end of 3 weeks.
Week
1
2
3
4
5
6
Forecast Demand
75
50
30
40
70
20
Customer Orders
80
45
40
50
50
5
70
25
85
35
65
45
Projected Available Balance
50
ATP
25
10
45
MPS
100
100
100
Time Fences
Consider the product structure shown in Figure 3.9. Item A is a master-scheduled item and
is assembled from B, C, and D. Item D, in turn, is made from raw material E. The lead
times to make or to buy the parts are shown in parentheses. The lead time to assemble A
is 2 weeks. To purchase B and C, the respective lead times are 6 and 5 weeks. To make
D takes 8 weeks, and the purchase lead time for raw material E is 16 weeks. The longest
cumulative lead time is thus 26 weeks 1A + D + E = 2 + 8 + 16 = 26 weeks2.
Since the cumulative lead time is 26 weeks, the MPS must have a planning horizon of at
least 26 weeks. A planning time fence of less than 26 weeks (say, for example, at 24 weeks)
means that if a new MPS order is placed for week 24 (at the end of the planning horizon),
any order for component E would already be two weeks late to make the MPS in week 24.
Suppose that E is a long-lead-time electronic component and is used in the assembly of
other boards as well as D. When E is received 16 weeks after ordering, a decision must be
made to commit E to be made into a D or to use it in another board. In 8 weeks, a decision must
be made to commit D to the final assembly of A. The company would not have to commit the
E to making D until 10 weeks before delivery of the A. At each of these stages, the company
B
(LT = 6)
FigurE 3.9 Product structure.
A
(LT = 2)
C
(LT = 5)
D
(LT = 8)
E
(LT = 16)
58
Chapter three
commits itself to more cost and fewer alternatives. Therefore, the cost of making a change
increases and the company’s flexibility decreases as production gets closer to the delivery time.
The establishment or use of a demand time fence is essentially a management decision. Depending on the nature of the product and the possible flexibility of the manufacturing process, management may set a time zone to essentially “freeze” the MPS. If they
set that time zone (demand time fence), for example, at three weeks, then no additional
customer orders or changes to the MPS should be made without managerial analysis and
intervention. The advantage of using a demand time fence provides stability to the production process and will tend to minimize costs that might be associated with changing some
aspect of an order so close to completion. The obvious disadvantage is a loss of flexibility
and response during that time period. The costs and benefits should be evaluated prior to
making the decision to use a demand time fence.
Changes to the master production schedule will occur. For example,
Customers cancel or change orders.
Machines break down or new machines are added, changing capacity.
■■ Suppliers have problems and miss delivery dates.
■■ Processes create more scrap than expected.
■■
■■
A company wants to minimize the cost of manufacture and also be flexible enough
to adapt to changing needs. Changes to production schedules can result in the following:
Increased costs. Cost increases due to rerouting, rescheduling, extra setups, expediting,
and buildup of work-in-process inventory.
■■ Decreased customer service. A change in quantity of delivery can disrupt the schedule
of other orders.
■■ Reduced credibility. Loss of credibility for the MPS and the planning process.
■■
Changes that are far off on the planning horizon can be made with little or no cost
or disruption to manufacturing, but the nearer to delivery date, the more disruptive and
costly changes will be. To help in the decision-making process, companies establish zones
divided by time fences. Figure 3.10 shows how this concept might be applied to product A.
The zones and time fences are as follows:
Frozen zone. Capacity and materials are committed to specific orders. Since changes
would result in excessive costs, reduced manufacturing efficiency, and poor customer
service, senior management’s approval is usually required to make changes. The
extent of the frozen zone is defined by the demand time fence. Within the demand
time fence, demand is usually based on customer orders, not forecast.
■■ Slushy zone. Capacity and material are committed to a less extent. This is an area for
trade-offs that must be negotiated between marketing and manufacturing. In this zone,
materials have been ordered and capacity established; these are difficult to change.
However, changes in priorities are easier to change. The extent of the slushy zone is
defined by the planning time fence. Within this time fence, the computer will generally not allow the automatic rescheduling of MPS orders. Any recommendation to
change an MPS order in this zone will need to be evaluated before changes are made.
Often, the minimum length of the planning time fence is determined by the cumulative
lead time of the product.
■■ Liquid zone. Any change can be made to the MPS as long as it is within the limits set
by the production plan. Changes are routine and are often made by the computer program without the need for input from the planner.
■■
0
FROZEN
Due
Date
Demand
Time Fence
FigurE 3.10 MPS and time fences.
SLUSHY
LIQUID
Planning
Time Fence
Master Scheduling
59
Changes to the MPS will occur. They must be managed and decisions made with full
knowledge of the costs involved.
Error Management
Errors in customer orders occur all the time and require constant attention. Three general
types of errors occur:
1. Wrong product or specification.
2. Wrong amount (too little or too much).
3. Wrong shipping date (too early or too late).
They require different responses, reengineering, alteration, negotiation of partial shipment, or expediting of shipment.
suMMary
The master production schedule is a plan for the production of individual end items. It
must match demand for the product in total, but it is not a forecast of demand. The MPS
must be realistic. It must be achievable and reflect a balance between required and available capacity.
The MPS is the meeting ground for sales and production. It provides a plan from
which realistic delivery promises can be made to customers. If adjustments have to be
made in deliveries or the booking of orders, they are done through the MPS.
Master production scheduling’s major functions are to form a link between the production plan and the facility, as an input to plan capacity, as the major input to MRP, to
help make order promises, keep priorities valid, and be a major link between production
and sales.
The MPS must be realistic and based on what production can and will do. If it is not,
the results will be as follows:
Overload or underload of plant resources.
Unreliable schedules resulting in poor delivery performance.
■■ High levels of work-in-process (WIP) inventory.
■■ Poor customer service.
■■ Loss of credibility in the planning system.
■■
■■
Key terMs
Available to promise (ATP) 54
Planning time fence 58
Demand time fence 56
Final assembly schedule (FAS) 51
Frozen zone 58
Liquid zone 58
Master production schedule (MPS) 45
Postponement 52
Projected available balance (PAB) 56
Rough-cut capacity planning 49
Scheduled receipt 54
Slushy zone 58
Questions
1. What four functions does the master production schedule (MPS) perform in the production
planning system?
2. What functions does the MPS perform between sales and production?
3. Does the MPS work with families of products or with individual items?
60
Chapter three
4. Where does the information come from to develop an MPS?
5. What are the three steps in making an MPS?
6. What is the purpose of a rough-cut capacity plan?
7. Where is the resource bill used?
8. At what level should master production scheduling take place?
a. In a make-to-stock environment?
b. In a make-to-order environment?
c. In an assemble-to-order environment?
9. What is a final assembly schedule (FAS)? What is its purpose?
10. What is a planning horizon? What decides its minimum time? Why would it be longer?
11. How do the production plan and the MPS relate to sales and to the sales forecast?
12. What is the ATP (available to promise)? How is it calculated?
13. What is the purpose of time fences? Name and describe the three main divisions.
14. What would happen if the planning horizon for the master schedule were too short? Why?
15. What potential problem might arise if time fences are not used? Why?
16. What types of production environments might use both the FAS and the MPS? Why?
probleMs
3.1. The Wicked Witch Whisk Company manufactures a line of broomsticks. The most
popular is the 36-inch model, and the sales department has prepared a forecast for 6
weeks. The opening inventory is 30. As master scheduler, you must prepare an MPS.
The brooms are manufactured in lots of 100.
Week
1
2
3
4
5
6
Forecast Sales
10
50
25
50
10
15
Projected Available Balance
30
MPS
Answer.
There should be scheduled receipts in weeks 2 and 4.
3.2. The Shades Sunglass Company assembles sunglasses from frames, which it makes,
and lenses, which it purchases from an outside supplier. The sales department has
prepared the following 6-week forecast for Ebony, a popular model. The sunglasses
are assembled in lots of 220, and the opening inventory is 300 pairs. Complete the
projected available balance and the master production schedule.
Week
1
2
3
4
5
6
Forecast Sales
200
300
300
200
150
150
Projected Available Balance
MPS
300
Master Scheduling
61
3.3. The Amalgamated Mailbox Company manufactures a family of two mailboxes. The
production plan and the MPS are developed on a quarterly basis. The forecast for the
product group follows. The opening inventory is 270 units, and the company wants to
reduce this to 150 units at the end of the year. Develop a level production plan.
production plan
Quarter
1
2
3
4
Forecast Sales
220
300
200
200
Projected Available Balance
Total
270
Production Plan
Answer.
Quarterly production = 200 units.
The forecast sales for each of the mailboxes in the family also follow. Develop
an MPS for each item, bearing in mind that production is to be leveled as in the production plan. For each mailbox, the lot size is 220.
Mailbox A. Lot size: 200
Quarter
1
2
3
4
Forecast Sales
120
180
100
120
Projected Available Balance
Total
120
MPS
Answer.
Scheduled receipts in quarters 2 and 3.
Mailbox B. Lot size: 200
Quarter
1
2
3
4
Forecast Sales
100
120
100
80
Projected Available Balance
Total
150
MPS
Answer.
Scheduled receipts in quarters 1 and 4.
3.4. Worldwide Can-Openers, Inc., makes a family of two hand-operated can openers.
The production plan is based on months. There are 4 weeks in this month. Opening
inventory is 2000 dozen, and it is planned to increase that to 4000 dozen by the end
of the month. The MPS is made using weekly periods. The forecast and projected
62
Chapter three
available balance for the two models follow. The lot size for both models is 1000
dozen. Calculate the production plan and the MPS for each item.
production plan
Week
1
2
3
4
Forecast
3000
3500
3500
4000
Week
1
2
3
4
Forecast
2000
2000
2500
2000
Week
1
2
3
4
Forecast
1000
1500
1000
2000
Projected Available Balance
Total
2000
Production Plan
Model A
Projected Available Balance
Total
1500
MPS
Model B
Projected Available Balance
Total
500
MPS
3.5. In the example given on page 47 earlier in the chapter, the MPS was unsatisfactory
because there were poor inventory balances compared to the production plan. There
was also a stockout for product C in periods 2 and 3. Revise the production plans for
the three products to cut out or reduce these problems.
3.6. The Acme Widget Company makes widgets in two models, and the bottleneck operation is in work center 10. Following is the resource bill (in hours per part).
Hours per Part
Work Center
Model A
Model B
10
2.4
3.5
Master Scheduling
63
The master production schedule for the next 5 weeks is
Week
1
2
3
4
5
Model A
70
50
50
60
48
Model B
20
40
55
30
45
a. Using the resource bill and the master production schedule, calculate the number of
hours required in work center 10 for each of the 5 weeks. Use the following table to
record the required capacity on the work center.
Week
1
2
3
4
5
Model A
Model B
Total Hours
Answer.
The total hours required are as follows: week 1, 238; week 2, 269; week 3,
313; week 4, 249; and week 5, 273.
b. If the available capacity at work station 10 is 260 hours per week, suggest possible
ways of meeting the demand in week 3.
3.7. Calculate the available to promise using the following data. There are 90 units on
hand.
Week
1
2
3
4
5
Customer Orders
70
70
20
40
10
MPS
100
100
6
100
ATP
Answer.
ATP in week 1, 30; week 2, 10; week 4, 50; and week 6, 100.
3.8. Given the following data, calculate how many units are available to promise. There
are 5 units on hand.
Week
1
Customer Orders
MPS
ATP
2
3
4
21
17
8
30
30
30
5
6
3
64
Chapter three
3.9. Using the scheduled receipts, calculate the ATP. There are zero units on hand.
Week
1
2
Customer Orders
10
MPS
50
3
4
10
5
6
60
16
7
8
9
10
10
50
50
ATP
3.10. Using the scheduled receipts, calculate the ATP. There are 45 units on hand.
Week
1
2
3
4
5
6
7
8
Customer Orders
45
50
35
40
30
40
20
18
MPS
100
100
100
100
ATP
3.11. Calculate the available to promise using the following data. There are 50 units on
hand.
Week
1
2
3
4
5
6
Customer Orders
20
50
30
30
50
30
MPS
100
100
ATP
3.12. Given the following data, can an order for 20 for delivery in week 4 be accepted?
Calculate the ATP using the following table. On hand = 50 units.
Week
1
2
3
4
5
6
7
8
Customer Orders
50
50
30
40
50
40
30
15
MPS
100
100
100
100
ATP
Answer.
Yes. Ten can come from the ATP for week 4 and 10 from the ATP for
week 2.
Master Scheduling
65
3.13. Given the following data, can an order for 40 more units for delivery in week 5 be
accepted? If not, what do you suggest can be done? There are zero units on hand.
Week
1
2
3
4
5
6
7
8
Customer Orders
70
10
50
40
10
15
20
15
MPS
100
100
100
ATP
3.14. Given the following data, calculate the projected available balance and the planned
MPS receipts. The lot size is 200. The demand time fence is 2 weeks.
Week
1
2
3
4
Forecast
80
80
80
70
Customer Orders
100
90
50
40
Projected Available Balance
140
MPS
Answer.
There is a planned MPS receipt in week 2.
3.15. Given the following data, calculate the projected available balance and the planned
MPS receipts. The lot size is 100. The demand time fence is 2 weeks.
Week
1
2
3
4
Forecast
50
50
50
50
Customer Orders
60
30
65
25
Projected Available Balance
60
MPS
3.16. Complete the following problem. The lead time is one week and the demand time
fence is the end of week 3. There are 20 on hand. The lot size is 60.
Period
1
2
3
4
5
6
Forecast
20
21
22
20
28
25
Customer Orders
19
18
20
18
30
22
Projected Available Balance
MPS
ATP
20
66
Chapter three
3.17. Product A is an assemble-to-order product. It has a lot size of 150, and currently
has an on-hand inventory of 110 units. There is a 2-week demand time fence and a
12-week planning time fence. The following table gives the original forecast and the
actual customer orders for the next 12 weeks:
Week
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
80
80
80
70
70
70
70
70
70
70
70
70
Demand
83
78
65
61
49
51
34
17
11
7
0
0
a. Given this information, develop a realistic master schedule, complete with ATP
logic.
b. Tell how you would respond to each of the following customer order requests.
Assume these are independent requests, and do not have cumulative effects.
■■ 20 units in week 3
■■ 40 units in week 5
■■ 120 units in week 7
3.18. a. Given the following master schedule, fill in the projected available and available
to promise rows:
On hand: 35
Planning time fence: 10
Lot size: 200
Demand time fence: 2
Period
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
30
40
40
50
40
40
30
40
40
50
40
40
Customer Orders
31
35
29
21
17
14
33
11
5
2
0
1
Projected Available
Available to Promise
MPS
200
200
b. A customer wants an order of 100 in period 4. What can you tell him?
c. The customer from part (b) cancels his request, but then says he wants 120 in period 5.
What do you tell him now?
d. Sales has requested that you add an MPS of 200 in period 9 to cover their needs for
a sales promotion. What do you tell them and why?
e. What action (if any) should be taken in period 11? Why is it okay to take the
action?
case study 3.1
Acme Water Pumps
The Acme Water Pump company has a problem. The pumps are fairly expensive to
make and store, so the company tends to keep the inventory low. At the same time, it is
important to respond to demands quickly, since a customer who wants a water pump is
very likely to get one from a competitor if Acme doesn’t have one available immediately.
Acme’s current policy to produce pumps is to produce 100 per week, which is the average demand. Even this is a problem, as the production manager has pointed out, since the
equipment is also used for other products and the lot size of 300 would be much more
efficient. He said he is currently set up for water pump production for the next week and
states that he has capacity available to produce 300 at a time next week.
Master Scheduling
67
The following lists the forecasts and actual customer orders for the next 12 weeks
Week
Forecast
1
90
2
120
3
110
4
80
5
85
6
95
7
100
8
110
9
90
10
90
11
100
12
110
Customer Orders
105
97
93
72
98
72
53
21
17
6
2
5
The president of Acme has said that he wants to consider using a formal MPS with
ATP logic to try to meet demand more effectively without a large impact on inventory.
Acme has decided to use a demand time fence at the end of week 3 and has also found out
that its current inventory is 25 units. Assume Acme will use the MPS lot size of 300 and
that it will produce the first of those lots in week 1.
assignment
1. Develop a master schedule using the information above.
2. A customer has just requested a major order of 45 pumps for delivery in week 5.
What would you tell the customer about having such an order? Why? What, if anything, would such an order do to the operation?
case study 3.2
The MasterChip Electronics Company
Sally Jackson, production manager of the MasterChip Electronics Company, was having another frustrating day. The final assembly area was woefully behind schedule, and
several large orders were several days, and some several weeks, behind the promised delivery date. Customers were not happy and were giving lots of angry messages to the sales
force. At the same time, some of the work areas in the early portions of the production
process apparently did not have enough work. Sally viewed this as an equally important
issue, since she could think of only two possible solutions—either let the people stand
around and do nothing or have them work ahead on some of the components even though
no order existed for those components. Working ahead was risky because their products
competed in a market where customers could demand a lot of options for a basic product,
and some of those options had highly variable demand (one option, for example, could go
for months with no demand and then all at once have a very large demand as one customer
ordered a large number of a product with that option). That was not likely to change since
most of their customers were large retail chain stores. Letting people stand around was
also bad, since she was evaluated on labor efficiency and utilization, and a worker not
working would make those numbers look very bad.
She would like to be able to send some of the workers home for a day or part of a
day, but the local union agreement prohibited that. She also liked to think about the possibility of using some of those workers to help out in another area (final assembly, in this
current situation), but the union agreement also had specific work classifications for each
worker, and those could not be violated. Even if that were possible, she knew it could be
a problem since most of the production workers in the area with little work knew almost
nothing about how the final assembly area worked, and that could generate lots of quality
problems.
Sally made a note to herself to develop some specific numbers for her weekly meeting with the human resources manager. Every week she looked at the demand for each
area and put together a set of recommendations for laying off some workers in one area
and calling back some workers for another area. She knew that was allowed, on a week by
week basis, under the union contract, but she still hated that task. Even though she could
usually come up with some good numbers, she could not neglect the following impacts:
■■
These workers often were the sole source of income for their families, and even a
week of layoff would likely imply hardships on their families.
68
Chapter three
The longer a worker was not working, their skills were not allowed to remain at a high
level of effectiveness. When they returned, they typically would not be able to work as
efficiently as before, and also represented the potential for a larger number of quality
problems.
■■ Even if they remained effective (if, for example, they had only been gone for a week),
it was highly likely they would be resentful of the layoff, and why should they feel
loyalty to the company when the company had not been loyal to them? The feelings of
resentment might make them less efficient on purpose.
■■ Many of their best workers had skills that were in demand by several other companies.
Why should a highly skilled worker with those skills in demand put up with those
occasional layoffs when they had other choices? Just in the last few months, she had
lost more than 10 of her best workers by having them go to work for one of the competitors of MasterChip.
■■
Just as she was starting to work on the numbers for her meeting with the human resources manager, Andy Morgan (the sales manager) came into her office. The conversation went like this:
andy: “Sally, I’ve got some good news and some bad news for you. First, the good
news: I just got off the phone with the buyer for Ajax Department Stores. They
want a very large order of over 1000 of the A77 product. They have some sort of
promotion in the works and that product is to be featured.”
sally: “When did you promise them that we would have the order done?”
andy: “I gave them our standard lead time for the product, six weeks.”
sally: “That’s going to be a problem for us. The A77 uses a power supply that is
somewhat expensive, so we have only about 200 in stock. It generally takes us
8–10 weeks to get those in from our supplier. I suppose we could expedite a
shipment, but that supplier would demand a much higher price since it disrupts
their own operation so much to do an expedite. It might cost us enough extra to
almost eliminate any profit on the order for us.”
andy: “Why don’t you people keep enough inventory—you know ours is a competitive business and we have to be responsive to our customers? If we can’t make
this order in six weeks, we are messing with a planned promotion from a major
retail chain, and they won’t be at all pleased. I wouldn’t be a bit surprised if they
started buying from one of our competitors. That point brings me to the bad
news: I’m getting lots of angry phone calls about those orders you have behind
schedule in final assembly. Remember, the customers of our customers tend to
walk out of a store that doesn’t have a product they want and go to a different
store. Our customers are very sensitive to having their orders shipped on time.
Can’t your production people get your act together?”
sally: “You should know that we can’t keep a lot of inventory sitting around. It is
expensive to hold, since electronics are easily subject to being damaged in storage, and as the technology changes so fast it also may become obsolete before
we can even use it. Management would not like it too well if our inventory
expense kills all our possible profit. Also, you taking an order like this without
checking first if we can do it, is kind of stupid. It’s that kind of thing that causes
the problems we have.”
andy: “Sally, that’s just silly. I have a customer on the phone that wants to spend a lot
of money with us for a big order. How do you think it would sound if I told them
to wait while I get permission from someone else to take the order? We can’t mess
around like that in sales; we need to work hard to get orders, and we did quote the
standard lead time we give all our customers for that A77 product. You people have
to work better. We can do our job to sell it, why can’t you do your job to make it?”
All Sally could do after that conversation was to search for a pain killer for her newly
developed headache, knowing she had to deal with that before she started to think on how
she should deal with the problems she had in addition to the new one that was just handed
to her by Andy.
Master Scheduling
69
assignment
What are the key issues in this case? Be sure to classify them as much as possible as symptoms versus core causes. Be sure to keep in mind the constraints as defined by the type of
customer and the internal conditions. Once you have analyzed and classified the issues,
develop a comprehensive solution for MasterChip that can deal more effectively with their
situation.
case study 3.3
Macarry’s Bicycle Company
Macarry’s Bicycle Company makes and sells high-quality bicycles, primarily to larger
North American bicycle retail outlets and to some wholesalers for smaller retail shops.
They have several models, and most of those models have a fairly large number of options
that can be mixed for a very large number of possible designs. The bicycles, for example,
can be made in a number of colors, type of seats, number of speed settings (gears), type
of tires and wheels, type of brakes, and handle bar styles. In addition, there are several options that can be included or not, including headlights and taillights, water bottle carriers,
baskets, or kickstands.
In such an environment, it is clearly difficult to know how much inventory to carry
or produce for each of the options. Several years ago the company decided they could not
establish a master schedule for each combination of all the options. That would literally
imply creating thousands of master schedules (one for each type of bicycle that is possible
to make), and some of those combinations might, in fact, never be ordered. Instead they
decided to make a master schedule for each of the options for a bicycle model and another
one for the common parts for the model (for example, a particular model has only one
frame, and most of the connectors are common, such as nuts and bolts). The common part
forecast was based on the total number of bicycles of a model type they planned to sell in
a given period, and that allowed them to calculate a forecast for each of the options based
on the historical percentage of the model sales that requested that option. Using this approach a final bicycle would never be produced except to a specific order from a specific
customer.
The cumulative lead time to obtain or make all the parts for the bicycle was 20 weeks,
so that is what the company used as their planning time fence. This is important to know
since in many parts of the country the sales of bicycles were very seasonal. Bicycle shops
in the North sold very few in winter, but in spring the demand was very high. The bicycle
shops did not like to keep a lot of inventory of finished bicycles because of the cost and
the fact that they did not know from year to year which type of options might be popular.
They tended to wait as long as possible to place an order, but then were very sensitive
that the order would be delivered in a timely manner. When a customer wanted to buy a
bicycle they did not want to wait, especially since the season was short in some parts of
the country.
The following charts show the forecast for one bicycle model (a hybrid heavy-duty
bicycle), existing confirmed orders from customers, existing inventory, and master production schedule quantities of the common parts and a few of the options for the next
12 weeks. The forecasts for options are computed as follows: The 18-speed gear option
was historically selected for this model bicycle 70% of the time. Since the forecast for
this model of bicycle for the first week was 50, the forecast for the gear options could be
calculated as 35 (70% of 50). The historical percentage of demand for the straight handlebars was 30%, and historically 20% of the orders included the head and tail light set. To
understand the orders, for example, the first week there were orders for 56 of this bicycle
model—37 of those orders wanted the 18-speed option, 16 of those orders wanted the
straight handlebars, and 2 of those order wanted the light set. This data was taken from the
late winter/early spring time frame, when the demand for the bicycles was starting to grow
as bicycle shops started to prepare for their heavy sales period.
70
Chapter three
Common parts (Frame, etc.) Existing inventory 40
Week
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
50
55
60
62
65
65
68
70
75
75
80
85
Cust. Orders
56
52
45
33
70
50
35
60
20
20
0
0
MPS
200
200
200
200
18-speed gear option Existing inventory 25
Week
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
35
39
42
44
46
46
48
49
53
53
56
60
Cust. Orders
37
38
40
33
50
20
25
40
5
5
0
0
MPS
150
150
150
150
Straight handlebars Existing inventory 20
Week
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
15
17
18
19
20
20
21
21
23
23
24
26
Cust. Orders
16
18
20
5
15
22
15
20
5
8
0
0
MPS
60
60
60
60
Head and tail light set Existing inventory 5
Week
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
10
11
12
13
13
13
14
14
15
15
16
17
Cust. Orders
2
12
10
8
15
9
7
11
2
1
0
MPS
30
30
30
30
30
0
30
assignment
1. Fill in the master schedules listed below, taking the data from the above tables and
adding row values for projected inventory and available to promise (ATP). Assume
there is no demand time fence for this data.
2. Once you have completed the tables, examine the list of orders for this model bicycle and
determine specifically what information should be given to the perspective customer. For
example, if the order request was for 40 bicycles (with defined options) in week 4 and
it appears that only 32 could be delivered, then you should be telling the customer that
they can have only 32 in week 4 and then the rest could be delivered at a later week (you
should be specific as to WHICH week). Assume the orders listed for evaluation are NOT
cumulative—in other words, evaluate each requested order independently ignoring the
existence of the other requested orders for the evaluation of this one order.
3. Suppose Macarry’s Bicycle managers discover that a major competitor has had to shut
down their production for the next three months due to a major fire. The Macarry’s
managers fully expect that many of the competitor’s customers will turn to Macarry’s
Bicycles to fill their orders during this critical time for them. In fact, one of the competitor’s customers has already asked about an order of 250 of the models for delivery
in week 5. What actions should Macarry’s take in this case? Be as specific as possible.
Common parts (Frame, etc.). Existing inventory 40
Week
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
50
55
60
62
65
65
68
70
75
75
80
85
Cust. Orders
56
52
45
33
70
50
35
60
20
20
0
0
Projected Inven.
MPS
ATP
200
200
200
200
Master Scheduling
71
18-speed gear option Existing inventory 25
Week
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
35
39
42
44
46
46
48
49
53
53
56
60
Cust. Orders
37
38
40
33
50
20
25
40
5
5
0
0
Projected Inven.
MPS
150
150
150
150
ATP
Straight handle-bars Existing inventory 20
Week
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
15
17
18
19
20
20
21
21
23
23
24
26
Cust. Orders
16
18
20
5
15
22
15
20
5
8
0
0
Projected Inven.
MPS
60
60
60
60
ATP
Head and tail light set Existing inventory 5
Week
1
2
3
4
5
6
7
8
9
10
11
12
Forecast
10
11
12
13
13
13
14
14
15
15
16
17
Cust. Orders
2
12
10
8
15
9
7
11
2
1
0
0
Projected Inven.
MPS
30
30
30
30
30
30
ATP
Here are the orders to evaluate. Again you are reminded to treat these independently. For
example, when you evaluate order number 2, ignore the other order requests (1, 3, and 4),
and so forth.
a. A customer is asking about an order of 32 of the bicycles for week 3. All 32 are to be
18 speed, 12 are to have straight handle-bars, and 14 are to have the light set.
b. A customer is asking about an order of 60 of the bicycles for week 6. Fifty of them are
to be 18 speed, 12 are to have straight handlebars, and 5 to have the light set.
c. A customer is asking about an order of 20 of the bicycles for week 2. All are to be 18
speed, all are to have straight handlebars, and all are to have the light set.
d. A customer is asking about an order of 110 of the bicycles in week 7. Sixty are to be
18 speed, 22 are to be straight handlebars, and 15 are to have the light set.
Chapter
four
MaterIal requIreMents
PlannIng
IntroductIon
Chapter 3 described the role of the master production schedule (MPS) in showing the end
items, or major components, that manufacturing intends to build. These items are made or
assembled from components that must be available in the right quantities and at the right
time to meet the MPS requirements. If any component is missing, the product cannot be built
and shipped on time. Material requirements planning (MRP) is the system used to avoid
missing parts. It establishes a schedule (priority plan) showing the components required at
each level of the assembly and, based on lead times, calculates the time when these components will be needed.
This chapter will describe bills of material (the major building block of material requirements planning), detail the MRP process, and explain how the material requirements plan is
used. But first, some details about the environment in which MRP operates will be discussed.
Nature of Demand
There are two types of demand: independent and dependent. Independent demand is not
related to the demand for any other product. For example, if a company makes wooden
tables, the demand for the tables is independent—it is essentially independent of any actions
taken internally in the company. Instead, it is dependent only on the external demand for the
table. Master production schedule items are independent demand items. The demand for the
sides, ends, legs, and tops depends on the demand for the tables, and these are dependent
demand items.
Figure 4.1 depicts a product tree that shows the relationship between independent and
dependent demand items. The figures in parentheses show the required quantities of each
component.
Since independent demand is not related to the demand for any other assemblies or
products, it must be forecast. However, since dependent demand is directly related to the
demand for higher-level assemblies or products, it can be calculated. Material requirements planning is designed to do this calculation.
An item can have both a dependent and an independent demand. A service or replacement part has both. A manufacturer of vacuum cleaners uses flexible hose in the assembly of
the units. In the assembly of the vacuums, the hose is a dependent demand item. However,
the hose has a nasty habit of breaking, and the manufacturer must have replacement hoses
available. Demand for replacement hoses is independent since demand for them does not
depend directly upon the number of vacuums manufactured.
Independent Demand
(Forecast)
Table
Legs
(4)
Ends
(2)
Figure 4.1 Product tree.
72
Sides
(2)
Top
(1)
Hardware
Kit
(1)
Dependent Demand
(Calculated)
Material requirements Planning
73
Dependency can be horizontal or vertical. The dependency of a component on its
parent is vertical. However, components also depend on each other. If one component
is going to be a week late, then the final assembly is a week late. The other components
are not needed until later. This is also a dependency and is called horizontal dependency.
Planners are concerned with horizontal dependency when a part is delayed or there is a
shortage, for then other parts will have to be rescheduled.
Objectives of MRP
Material requirements planning has two major objectives: determine requirements and
keep priorities current.
Determine requirements The main objective of any manufacturing planning and
control system is to have the right materials in the right quantities available at the right
time to meet the demand for the firm’s products. The material requirements plan’s objective is to determine what components are needed to meet the master production schedule
and, based on lead time, to calculate the periods when the components must be available.
It must determine the following:
What to order.
How much to order.
■■ When to order.
■■ When to schedule delivery.
■■
■■
Keep priorities current The demand for, and supply of, components changes daily.
Customers enter or change orders. Components get used up, suppliers are late with delivery, scrap occurs, orders are completed, and machines break down. In this ever-changing
world, a material requirements plan must be able to reorganize priorities to keep plans current. It must be able to add and delete, expedite, delay, and change orders.
Linkages to Other Manufacturing Planning
and Control Functions
The master production schedule drives the material requirements plan. The MRP is a priority plan for the components needed to make the products in the MPS. The plan is valid
only if capacity is available when needed to make the components, and the plan must be
checked against available capacity. The process of doing so is called capacity requirements planning and is discussed in Chapter 5.
Material requirements planning drives, or is an input to, production activity control
(PAC) and purchasing. MRP plans the release and receipt dates for orders. PAC and purchasing must plan and control the performance of the orders to meet the due dates.
Figure 4.2 shows a diagram of the production planning and control system with its
inputs and outputs.
MRP Software
If a company makes a few simple products, it might be possible to perform material
requirements planning manually. However, most companies need to keep track of thousands of components in a world of changing demand, supply, and capacity.
In the days before computers, it was necessary to maintain extensive manual systems
and to have large inventories and long lead times. These were needed as a cushion due to the
lack of accurate, up-to-date information and the inability to perform the necessary calculations quickly. Somehow, someone in the organization figured out what was required sooner
or, very often, later than needed. “Get it early and get lots of it” was a good rule then.
Computers are incredibly fast, accurate, and ideally suited for the job at hand. With their
ability to store and manipulate data and produce information rapidly, manufacturing now has
a tool to use modern manufacturing planning and control systems properly. There are many
MRP software programs available, and while they may have some different looks, the processing logic is well-defined and tends to be the same for each of the different available programs.
74
Chapter four
INPUT
Business Plan
Financial Plan
Marketing Plan
Capacity
Production Plan
Forecasts
Customer Orders
Inventory
Capacity
MPS
Bill of Materials
Inventory
Capacity
OUTPUT
PRODUCTION
PLAN
MASTER
PRODUCTION
SCHEDULE
MATERIAL
REQUIREMENTS
PLAN
PURCHASING
Aggregate Plan
• By-Product Groups
• Inventory Levels
Detailed Plan
• By Week
• By End Item
Time-Phased
Manufacturing and
Purchase Orders
• For Raw Material
• For Components
PRODUCTION
ACTIVITY
CONTROL
Figure 4.2 Production planning and control system.
Inputs to the Material Requirements Planning System
There are three primary inputs to MRP systems:
1. Master production schedule.
2. Inventory records.
3. Bills of material.
Master production schedule. The master production schedule is a statement of
which end items are to be produced, the quantity of each, and the dates they are to be
completed. It drives the MRP system by providing the initial input for the items needed.
Inventory records. A major input to the MRP system is inventory. When a calculation
is made to find out how many are needed, the quantities available must be considered.
There are two kinds of information needed. The first is called planning factors
and includes information such as order quantities, lead times, safety stock, and scrap.
This information does not change often; however, it is needed to plan what quantities
to order and when to order for timely deliveries.
The second kind of information necessary is the status of each item. The MRP
system needs to know how much is available, how much is allocated, and how much
is available for future demand. This type of information is dynamic and changes with
every transaction that takes place.
This data is maintained in an inventory record, also called a part master or item
master. Each item has a record, and all the records together form a file or table.
Bills of material. The bill of material is one of the most important documents in a
manufacturing company. It is discussed next.
BIlls of MaterIal
Before something can be made, the components needed to make it must be known. To bake
a cake, a recipe is needed. To mix chemicals together, a formula is needed. To assemble a
wheelbarrow, a parts list is needed. Even though the names are different, recipes, formulas,
and parts lists tell what is needed to make the end product. All of these are bills of material.
APICS Dictionary, 14th edition defines a bill of material as “a listing of all the
subassemblies, intermediates, parts, and raw materials that go into making the parent
assembly showing the quantities of each required to make an assembly.” Figure 4.3 shows
Material requirements Planning
75
Description:
TABLE
Part Number: 100
Part
Number
Description
Quantity
Required
203
411
622
023
722
Wooden Leg
Wooden Ends
Wooden Sides
Table Top
Hardware Kit
4
2
2
1
1
Figure 4.3 Simplified bill of material.
a simplified bill of material. There are three important points around bills of material and
part numbers:
1. The bill of material shows all the parts required to make one of the item.
2. Each part or item has only one part number. A specific number is unique to one part
and is not assigned to any other part. Thus, if a particular number appears on two different bills of material, the part so identified is the same.
3. A part is defined by its form, fit, or function. If any of these change, then it is not the
same part and it must have a different part number. For example, a part when painted
becomes a different part and must have a different number. If the part could be
painted in three different colors, then each must be identified with its unique number.
The bill of material shows the components that go into making the parent. It does not
show the steps or process used to make the parent or the components. That information is
recorded in a routing. This is discussed in Chapters 5 and 6.
Bills of Material Structure
Bills of material structure refer to the overall design for the arrangement of bills of material files. Different departments in a company use bills of material for a variety of purposes.
Although each user has individual preferences for the way the bill should be structured, there
must be only one structure, and it should be designed to satisfy most needs. However, there can
be several formats, or ways, to present the bill. Following are some possible formats for bills.
Product tree Figure 4.4 shows a product tree for the bill of material shown in Figure 4.3.
The product tree is a convenient way to think about bills of material, but it is seldom used
except for teaching and testing. In this text, it is used for that purpose.
Parent–component relationship The product tree and the bill of material shown in
Figures 4.1 and 4.3 are called single-level structures. An assembly is considered a parent,
and the items that comprise it are called its component items. Figure 4.4 shows the parent–component relationship of the table (P/N 100). Unique part numbers have also been
assigned to each part. This makes identification of the part absolute.
Table
100
Legs (4)
203
Ends (2)
411
Figure 4.4 Product tree.
Sides (2)
622
PARENT
Top (1)
023
Hardware
Kit (1)
722
COMPONENT
76
Chapter four
Table
100
Base
200
Legs (4)
203
Top
023
Leg Bolts (4)
220
Sides (2)
622
Frame (1)
300
Ends (2)
411
Boards (3)
030
Leg
Supports (4)
533
Glue
066
Glue
066
Figure 4.5 Multilevel bill.
Multilevel bill Figure 4.5 shows the same product as the single-level bill shown in
Figures 4.3 and 4.4. However, the single-level components have been expanded into their
components.
Multilevel bills are formed as logical groupings of parts into subassemblies based
on the way the product is assembled. For example, a frame, chassis, doors, windows, and
engine are required to construct an automobile. Each of these forms a logical group of
components and parts and, in turn, has its own bill of material.
It is the responsibility of manufacturing engineering to decide how the product is to
be made: the operations to be performed, their sequence, and their grouping. The subassemblies created are the result of this. Manufacturing has decided to assemble the sides,
ends, and leg supports (part of the hardware kit) of the table (P/N 100) in Figure 4.4 into a
frame (P/N 300). The legs, leg bolts, and frame subassembly are to be assembled into the
base (P/N 200). The top (P/N 023) is to be made from three boards glued together. Note
that the original parts are all there, but they have been grouped into subassemblies and
each subassembly has its own part number.
One convention used with multilevel bills of material is that the last items on the tree
(legs, leg bolts, ends, sides, glue, and boards) are all purchased items. Generally, a bill of
material is not complete until all branches of the product structure tree end in a purchased
part or a raw material.
Each level in the bill of material is assigned a number starting from the top and working
down. The top level, or end product level, is level zero, and its components are at level one.
Multiple bill A multiple bill is used when companies usually make more than one
product, and the same components are often used in several products. This is particularly
true with families of products. Using our example of a table, the company makes two
models. They are similar except the tops are different. Figure 4.6 shows the two bills of
material. Because the boards used in the top are different, each top has a different part
number. The balance of the components are common to both tables.
Single-level bill A single-level bill of material contains only the parent and its
immediate components, which is why it is called a single-level bill. The tables shown in
Figure 4.6 have six single-level bills, and these are shown in Figure 4.7. Note that many
components are common to both tables.
The computer stores information describing the product structure as a single-level
bill. A series of single-level bills is needed to completely define a product. For example,
the table needs four single-level bills, one each for the table, base, top, and frame. These
can be chained together to form a multilevel, or indented, bill. Using this method, the
information has to be stored only once. For example, the frame (P/N 300) might be used
on other tables with different legs or tops.
Material requirements Planning
77
Table
100
Base
200
Legs (4)
203
Leg Bolts (4)
220
Sides (2)
622
Top
023
Frame (1)
300
Ends (2)
411
Glue
066
Boards (3)
030
Leg
Supports (4)
533
Glue
066
Table
150
Base
200
Legs (4)
203
Leg Bolts (4)
220
Sides (2)
622
Top
025
Frame (1)
300
Ends (2)
411
Boards (3)
035
Leg
Supports (4)
533
Glue
066
Glue
066
Figure 4.6 Multiple bills.
Table
100
Base
200
Table
150
Top
023
Base
200
Legs (4)
203
Top
025
Top
023
Leg Bolts (4)
220
Frame (1)
300
Boards (3)
030
Frame (1)
300
Sides (2)
622
Base
200
Ends (2)
411
Figure 4.7 Single-level bills.
Leg
Supports (4)
533
Glue
066
Top
025
Glue
066
Boards (3)
035
Glue
066
78
Chapter four
There are several advantages to using single-level bills, including the following:
Duplication of records is avoided. For instance, base 200 is used in both table 100 and
table 150. Rather than have two records of base 200, one in the bill for table 100 and
one in the bill for table 150, only one record need be kept.
■■ The number of records and the file size are reduced by avoiding duplication of
records.
■■ Maintaining bills of material is simplified. For example, if there is a change in base
200, the change needs be made in only one place.
■■
example Problem
Using the following product tree, construct the appropriate single-level trees. How
many Ks are needed to make 100 Xs and 50 Ys?
X
B(2)
F
Y
C
D
G(2)
J
M(2)
L
N
P
K(2)
M
N(2)
O
N(2)
O
G
J
K(2)
Answer
X
B(2)
Y
C
B
F
G(2)
D
L
D
L
M(2)
N
P
M
G
G
J
K(2)
Each X requires two Bs
Each B requires two Gs: 2 * 2
100 Xs require
=■■■■■■4 Gs for each X
=■400 Gs
Each Y requires one L
Each L requires one G: 1 * 1
50 Ys require
Total Gs required
=■■■■■■1 G
50 Gs
=■450
Each G requires two Ks
Total Ks required 2 * 450 =■900
Material requirements Planning
79
MANUFACTURING BILL OF MATERIAL
TABLE P/N 100
Part
Number
Description
Quantity
Required
200
203
220
300
622
411
533
066
023
030
066
Base
Legs
Leg Bolts
Frame
Sides
Ends
Leg Supports
Glue
Top
Boards
Glue
1
4
4
1
2
2
4
1
3
Figure 4.8 Indented bill of material.
Indented bill A multilevel bill of material can also be shown as an indented bill of
material. This bill uses indentations as a way of identifying parents from components.
Figure 4.8 shows an indented bill for the table in Figure 4.5.
The components of the parent table are listed flush left, and their components are
indented. The components of the base (legs, leg bolts, and frame) are indented immediately below their parents. The components of the frame are further indented immediately
below their parents. The components of the frame are further indented immediately below
their parents. Thus, the components are linked to their parents by indenting them as subentries and by listing them immediately below the parents.
Summarized parts list The bill of material shown in Figure 4.3 is called a summarized parts list. It lists all the parts needed to make one complete assembly. The parts list
is produced by the product design engineer and does not contain any information about the
way the product is made or assembled.
Planning bill A major use of bills of material is to plan production. Planning bills
are an artificial grouping of components for planning purposes. They are used to simplify
forecasting, master production scheduling, and material requirements planning. They do
not represent buildable products but an average product. Using the table example, suppose
the company manufactured tables with three different leg styles, three different sides and
ends, and three different tops. In total, they are making 2713 * 3 * 32 different tables,
each with its own bill of material. For planning purposes, the 27 bills can be simplified
by showing the percentage split for each type of component on one bill. Figure 4.9 shows
how the product structure would look. The percentage usage of components is obtained
from a forecast or past usage. Note that the percentage for each category of component
adds up to 100%.
Where-Used and Pegging Reports
Where-used report Where-used reports give the same information as bills of material, but the where-used report gives the parents for a component whereas the bill gives the
components for a parent. A component may be used in making several parents. Wheels
on an automobile, for example, might be used on several models of cars. A listing of all
the parents in which a component is used is called a where-used report. This has several
uses, such as in implementing an engineering change, or when materials are scarce, or in
costing a product.
80
Chapter four
Table
Common
Parts
100%
Legs
Sides
Tops
Leg A
40%
Side A
55%
Top A
45%
Leg B
35%
Side B
30%
Top B
30%
Leg C
25%
Side C
15%
Top C
25%
Figure 4.9 Planning bill.
Pegging report A pegging report is similar to a where-used report. However, the
pegging report shows only those parents for which there is an existing demand requirement, whereas the where-used report shows all parents for a component. The pegging
report shows the parents creating the demand for the components, the quantities needed,
and when they are needed. Pegging keeps track of the origin of the demand. Figure 4.10
shows an example of a product tree in which part C is used twice and a pegging report.
Uses for Bills of Material
The bill of material is one of the most widely used documents in a manufacturing company. Some major uses are as follows:
Product definition. The bill specifies the components needed to make the product.
Engineering change control. Product design engineers sometimes change the design
of a product and the components used. These changes must be recorded and controlled. The bill provides the method for doing so.
■■ Service parts. Replacement parts needed to repair a broken component are determined from the bill of material.
■■ Planning. Bills of material define what materials have to be scheduled to make the
end product. They define what components have to be purchased or made to satisfy
the master production schedule.
■■ Order entry. When a product has a very large number of options (e.g., cars), the
order-entry system very often configures the end product bill of materials. The bill
can also be used to price the product.
■■ Manufacturing. The bill provides a list of the parts needed to make or assemble
a product.
■■
■■
A
Pegged Requirements
Item
Number
C
B
C
Week
1
2
3
4
5
50
125
25
50
150
25
50
50
Source of Requirements
A
C
D
Figure 4.10 Pegged requirements.
B
50
25
100
100
Material requirements Planning
■■
81
Costing. Product cost is usually broken down into direct material, direct labor, and
overhead. The bill provides not only a method of determining direct material but also
a structure for recording direct labor and distributing overhead.
This list is not complete, but it shows the extensive use made of the bill of material
in manufacturing. There is scarcely a department of a company that will not use the bill at
some time. Maintaining bills of material and their accuracy is extremely important. Again,
the computer is an excellent tool for centrally maintaining bills and for updating them.
MaterIal requIreMents PlannIng Process
Each component shown on the bill of material is planned for by the material requirements
planning system. For convenience, it is assumed that each component will go into inventory and be accounted for. Whether the components actually go into a physical inventory
or not is unimportant. However, it is important to realize that planning and control take
place for each component on the bill. Raw material may go through several operations
before it is processed and ready for assembly, or there may be several assembly operations
between components and parent. These operations are planned and controlled by production activity control, not material requirements planning.
The purpose of material requirements planning is to determine the components
needed, quantities, and due dates so items in the master production schedule are made on
time. This section presents the basic MRP techniques for doing so. These techniques are
discussed under the following headings:
Exploding and offsetting
Gross and net requirements
■■ Releasing orders
■■ Capacity requirements planning
■■ Low-level coding and netting
■■ Multiple bills of material
■■
■■
Exploding and Offsetting
Consider the product tree shown in Figure 4.11. It is similar to the ones used before but
contains another necessary piece of information: lead time.
Lead time Lead time is the span of time needed to perform a process. In manufacturing it includes time for order preparation, queuing, processing, moving, receiving and
inspecting, and any expected delays. From the product tree shown in Figure 4.11, if B and
C are available, it will take 1 week to assemble A. Thus, the lead time for A is 1 week.
Similarly, if D and E are available, the time required to manufacture B is 2 weeks. The
purchase lead times for D, E, and C are all 1 week.
A
D
B
LT: 2 weeks
LT: 1 week
E
Figure 4.11 Product tree with lead time.
LT: 1 week
C
LT: 1 week
LT: 1 week
82
Chapter four
In this particular product tree, the usage quantities—the quantity of components needed
to make one of a parent—are all one. To make an A requires one B and one C, and to make
a B requires one D and one E.
Exploding the requirements Exploding is the process of multiplying the requirements by the usage quantity and recording the appropriate requirements throughout the
product tree.
Offsetting Offsetting is the process of placing the exploded requirements in their
proper periods based on lead time. For example, if 50 units of A are required in week 5,
the order to assemble the As must be released in week 4, and 50 Bs and 50 Cs must be
available in week 4.
Planned orders If it is planned to receive 50 of part A in week 5 and the lead time to
assemble an A is 1 week, the order will have to be released and production started no later
than week 4.
Thus, there should be a planned order receipt for 50 in week 5 and a planned order
release for that number in week 4. If an order for 50 As is to be released in week 4, 50 Bs
and 50 Cs must be available in that week. Thus, there must be planned order receipts for
those components in week 4. Since the lead time to assemble a B is 2 weeks, there must
be a planned order release for the Bs in week 2. Since the lead time to make a C is 1 week,
there must be a planned order release for 50 in week 3. The planned order receipts and
planned order releases for the Ds and Es are determined in the same manner. Figure 4.12
shows when orders must be released and received so the delivery date can be met. Note
that planned order releases and planned order receipts are paired with each other. Every
time you have a planned order receipt, it should generate a planned order release offset by
the lead time. Planned order releases and receipts are assumed to be orders for the item at
the beginning of each period.
Week
Part Number
A
B
C
D
E
1
2
3
Planned Order Receipt
Planned Order Release
5
50
50
Planned Order Receipt
Planned Order Release
50
50
Planned Order Receipt
Planned Order Release
50
50
Planned Order Receipt
Planned Order Release
50
Planned Order Receipt
Planned Order Release
50
Figure 4.12 Exploding and offsetting.
4
50
50
Material requirements Planning
83
example Problem
Using the product tree and lead times shown in Figure 4.11, complete the following
table to determine the planned order receipts and releases. There are 50 As required
in week 5 and 100 in week 6.
Week
Part Number
1
A
Planned Order Receipt
Planned Order Release
B
Planned Order Receipt
Planned Order Release
C
Planned Order Receipt
Planned Order Release
D
Planned Order Receipt
Planned Order Release
E
Planned Order Receipt
Planned Order Release
2
3
5
6
50
100
4
5
6
50
100
100
50
50
100
50
100
100
4
Answer
Week
Part Number
1
2
3
A
Planned Order Receipt
Planned Order Release
B
Planned Order Receipt
Planned Order Release
C
Planned Order Receipt
Planned Order Release
D
Planned Order Receipt
Planned Order Release
50
100
100
50
E
Planned Order Receipt
Planned Order Release
50
100
100
50
50
100
50
Gross and Net Requirements
The previous section assumed that no inventory was available for the As or any of the
components. Often inventory is available and must be included when calculating quantities to be produced. If for example, in the preceding problem, there are 20 As in stock, at
the beginning of period 5, only 30 need to be made. The requirements for component parts
would be reduced accordingly. The calculation is as follows:
Gross requirement = 50
Inventory available = 20
Net requirements = gross requirements - available inventory
Net requirements = 50 - 20 = 30 units
84
Chapter four
Since only 30 As need to be made, the gross requirement for Bs and Cs is only 30.
The planned order release of the parent becomes the gross requirement of the component.
The time-phased inventory record shown in Figure 4.12 can now be modified to
consider any inventory available. For example, suppose there are 10 Bs available as well
as the 20 As. The requirements for the components D and E would change. Figure 4.13
shows the change in the MRP record. Note that the projected available inventory shows
the quantity projected on hand for the end of the period.
example Problem
Complete the following table. Lead time for the part is 2 weeks. The order quantity
(lot size) is 100 units.
Week
1
2
3
4
50
45
20
1
2
3
4
75
50
25
45
80
20
100
20
60
Gross Requirements
Projected Available
75
Net Requirements
Planned Order Receipt
Planned Order Release
Answer
Week
Gross Requirements
Projected Available
75
Net Requirements
Planned Order Receipt
Planned Order Release
100
Releasing Orders
So far we have looked at the process of planning when orders should be released so work
is done in time to meet gross requirements. In many cases, requirements change daily.
A computer-based material requirements planning system automatically recalculates the
requirements for subassemblies and components and re-creates planned order releases to
meet the shifts in demand.
Planned order releases are just planned; they have not been released. It is the responsibility of the material planner to release planned orders, not the computer.
Since the objective of the MRP is to have material available when it is needed and
not before, orders for material should not be released until the planned order release date
arrives. Thus, an order is not normally released until the planned order is in the current
week (week 1).
Releasing an order means that authorization is given to purchasing to buy the necessary material or to manufacturing to make the component.
Before a manufacturing order is released, component availability must be checked.
The computer program checks the component inventory records to be sure that enough
material is available and, if so, to allocate the necessary quantity to that work order. If the
material is not available, the computer program will advise the planner of the shortage.
When the authorization to purchase or manufacture is released, the planned order
receipt is canceled, and a scheduled receipt is created in its place. For the example shown
in Figure 4.13, parts D and E have planned order releases of 20 scheduled for week 1.
These orders will be released by the planner, and then the MRP records for parts D and
E will appear as shown in Figure 4.14. Notice that scheduled receipts have been created,
replacing the planned order releases.
Material requirements Planning
85
Week
Part Number
A
B
C
D
E
Gross Requirements
Projected Available 20
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Projected Available 10
Net Requirements
Planned Order Receipt
Planned Order Release
1
2
3
4
20
20
20
20
Gross Requirements
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
50
0
30
30
30
10
10
10
30
0
20
20
20
Gross Requirements
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
5
30
0
30
30
0
30
0
20
0
20
20
20
0
20
0
20
20
20
Figure 4.13 Gross and net requirements.
Week
Part Number
D
E
1
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Figure 4.14 Scheduled receipts.
0
0
2
20
20
0
0
20
20
0
0
3
4
5
86
Chapter four
When a manufacturing order is released the computer will allocate the required
quantities of a parent’s components to that order. This does not mean the components
are withdrawn from inventory but that the projected available quantity is reduced. The
allocated quantity of components is still in inventory, but they are not available for other
orders. They will stay in inventory until withdrawn for use.
Scheduled receipts Scheduled receipts are orders placed on manufacturing or on a
vendor and represent a commitment to make or buy. For an order in a factory, necessary
materials are committed, and work-center capacity is allocated to that order. For purchased
parts, similar commitments are made to the vendor. The scheduled receipts row shows the
quantities ordered and when they are expected to be completed and available. They are
generally expected to be due at the start of the period for which they are scheduled.
Open orders Scheduled receipts on the MRP record are open orders on the factory or
a vendor and are the responsibility of purchasing and of production activity control. These
orders represent committed resources and are in process but not yet received. When the
goods are received into inventory and available for use, the order is closed out, and the
scheduled receipt disappears to become part of the on-hand inventory.
Net requirements The calculation for net requirements can now be modified to
include scheduled receipts.
Net requirements = gross requirements - scheduled receipts - available inventory
example Problem
Complete the following table. Lead time for the item is 2 weeks, and the order
quantity is 200. What action should be taken?
Week
1
2
3
4
Gross Requirements
Scheduled Receipts
Projected Available
150
Net Requirements
Planned Order Receipt
Planned Order Release
50
250
200
100
50
Week
1
2
3
4
Gross Requirements
Scheduled Receipts
Projected Available
150
Net Requirements
Planned Order Receipt
Planned Order Release
50
250
200
50
100
50
150
50
200
100
Answer
100
200
The order for 200 units should be released.
Basic MRP Record
Figure 4.15 shows a basic MRP record. There are several points that are important:
1. The current time is the beginning of the first period.
2. The top row shows periods, called time buckets. These are often a week but can be
any length of time convenient to the company. Today’s MRP applications typically
use daily time buckets.
Material requirements Planning
87
Week
Part Number
1
Gross Requirements
Scheduled Receipts
10
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
2
3
4
10
20
30
5
35
10
10
5
5
5
Figure 4.15 Basic MRP record.
3. The number of periods in the record is called the planning horizon, which shows the
number of future periods for which plans are being made. It should be at least as long
as the cumulative product lead time. Otherwise, the MRP system would not be able to
release planned orders of items at the lower level at the correct time.
4. An item is considered available at the beginning of the time bucket in which it is
required.
5. The quantity shown in the projected available row is the projected on-hand balance at
the end of the period.
6. The immediate or most current period is called the action bucket. A quantity in the
action bucket means that some action is needed now to avoid a future problem.
7. A bucketless system shows only the time buckets that have MRP activity, omitting
the time periods with no activity.
Capacity Requirements Planning
As in the previous planning levels discussed, the MRP priority plan must be checked
against available capacity. At the MRP planning level, the process is called capacity
requirements planning (CRP). Chapter 5 examines this activity in some detail. If the
capacity is available, the plan can proceed. If not, either capacity has to be made available
or the priority plans must be changed.
Low-Level Coding and Netting
A component may reside on more than one level in a bill of material. If this is the case, it is
necessary to make sure that all gross requirements for that component have been recorded
before netting takes place. Consider the product shown in Figure 4.16. Component C occurs
twice in the product tree and at different levels. It would be a mistake to net the requirements
for the Cs before calculating the gross requirements for those required for parent B.
Level
0
A
B
C
C
D
Figure 4.16 Multilevel product tree.
1
2
88
Chapter four
The process of collecting the gross requirements and netting can be simplified by
using low-level codes. The low-level code is the lowest level on which a part resides in all
bills of material. Every part has only one low-level code. The low-level codes for the parts
in the product tree shown in Figure 4.6 are
Part
Low-Level Code
A
B
C
D
0
1
2
2
Low-level codes are determined by starting at the lowest level of a bill of material and,
working up, recording the level against the part. If a part occurs again on a higher level, it
is not assigned a code because its existence on the lower level has already been recorded.
Once the low-level codes are obtained, the net requirements for each part can be
calculated using the following procedure. For the purpose of this exercise, there is a
gross requirement for part A of 50 in week 5, all lead times are 1 week, and the following
amounts are in inventory: A, 20 units; B, 10 units; and C, 10 units.
Procedure
1. Starting at level zero of the tree, determine if any of the parts on that level have a lowlevel code of zero. If so, those parts occur at no lower level, and all the gross requirements have been recorded. These parts can, therefore, be netted and exploded down to
the next level, that is, into their components. If the low-level code is greater than zero,
there are more gross requirements, and the part is not netted. In this example, A has a
low-level code of zero so there is no further requirement for As; it can be netted and
exploded into its components. Figure 4.17 shows the results.
2. The next step is to move down to level 1 on the product tree and to repeat the routine followed in step 1. Since B has a low-level code of 1, all requirements for B are
Week
LowLevel
Code
0
1
2
Part
Number
A
B
C
1
Gross Requirements
Scheduled Receipts
Projected Available 20
Net Requirements
Planned Order Receipt
Planned Order Release
2
3
4
50
20
20
20
20
30
Gross Requirements
Scheduled Receipts
Projected Available 10
Net Requirements
Planned Order Receipt
Planned Order Release
30
Gross Requirements
Scheduled Receipts
Projected Available 10
Net Requirements
Planned Order Receipt
Planned Order Release
30
Figure 4.17 Netting and exploding zero-level parts.
5
0
30
30
Material requirements Planning
89
Week
LowLevel
Code
1
2
2
Part
Number
B
C
D
1
Gross Requirements
Scheduled Receipts
Projected Available 10
Net Requirements
Planned Order Receipt
Planned Order Release
2
3
4
5
30
10
10
10
0
20
20
20
Gross Requirements
Scheduled Receipts
Projected Available 10
Net Requirements
Planned Order Receipt
Planned Order Release
20
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
20
30
Figure 4.18 Netting and exploding first-level parts.
recorded, and it can be netted and exploded. The bill of material for B shows that it is
made from a C and a D. Figure 4.18 shows the result of netting and exploding the Bs.
Part C has a low-level code of 2, which signifies there are further requirements for Cs
and at this stage they are not netted.
3. Moving down to level 2 on the product tree, part C has a low-level code of 2. This
signifies that all gross requirements for Cs are accounted for and that the process can
proceed and determine its net requirements. Notice there is a requirement for 30 Cs
in week 4 to be used on the As and a requirement of 20 Cs in week 3 to be used on
the Bs. Looking at its bill of material, it shows it is a purchased part and no explosion
is needed.
Figure 4.19 shows the completed material requirements plan. The process of levelby-level netting is now completed using the low-level codes of each part. The low-level
codes are used to determine when a part is eligible for netting and exploding. In this way,
each part is netted and exploded only once. There is no time-consuming re-netting and reexploding each time a new requirement is met.
Multiple Bills of Material
Most companies make more than one product and often use the same components
in many of their products. The material requirements planning system gathers the
planned order releases from all the parents and creates a schedule of gross requirements for the components. Figure 4.20 illustrates what happens. Part F is a component
of both C and B.
The same procedure used for a single bill of material can be used when multiple products are being manufactured. All bills must be netted and exploded level by level as was
done for a single bill.
90
Chapter four
Week
LowLevel
Code
Part
Number
0
1
1
2
5
20
20
20
0
30
30
30
10
10
10
0
20
20
20
10
10
10
20
30
0
10
10
30
0
30
30
20
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
D
4
30
Gross Requirements
Scheduled Receipts
Projected Available 10
Net Requirements
Planned Order Receipt
Planned Order Release
C
2
20
Gross Requirements
Scheduled Receipts
Projected Available 10
Net Requirements
Planned Order Receipt
Planned Order Release
B
3
50
Gross Requirements
Scheduled Receipts
Projected Available 20
Net Requirements
Planned Order Receipt
Planned Order Release
A
2
0
0
0
20
20
20
Figure 4.19 Completed material requirements plan.
B
F
C
(1)
G
(1)
E
Part B
Week
Planned Order
Release
(2)
F
(2)
2
3
30
30
Part C
1
20
2
3
Week
1
Planned Order
Release
20
Part F
Week
1
2
3
Gross
Requirements
20
80
60
Figure 4.20 Multiproduct MRP explosion.
Material requirements Planning
A
91
B
C(2)
F(2)
F(1)
Figure 4.21 Multiproduct tree.
Figure 4.21 shows the product trees for two products. Both are made from several
components, but, for simplicity, only those components containing an F are shown in the
product tree. Note that both have F as a component but at different levels in their product
tree. All lead times are 1 week. The quantities required are shown in parentheses; that is,
two Cs are required to make an A, one F is required to make a B, and two Fs are needed to
make a C. Figure 4.22 shows the completed material requirements plan that would result if
50 As were required in week 5 and 30 Bs in week 3.
Scrap is inherent in some processes due to errors or as a result of the process. As an
example, the loss due to bones, juice, and evaporation when cooking a turkey is estimated
at 50%. Industries that mix and pour liquids usually lose some product in the system. Due
to scrap as shown in these examples, extra production must be scheduled to produce the
desired end amount. To accommodate scrap MRP increases the planned order release from
the amount needed for the planned order receipt. For example, a process may generate
Week
LowLevel
Code
0
0
1
2
Part
Number
A
B
C
F
1
Gross Requirements
Scheduled Receipts
Projected Available 20
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available 10
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available 10
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Figure 4.22 Partial material requirements plan.
2
3
4
5
50
20
20
20
20
30
30
10
10
0
20
20
20
60
10
10
10
50
40
40
50
0
40
40
50
0
50
50
0
50
50
0
30
30
92
Chapter four
15% scrap. If the planned order receipt for an item is for 400 units, then the planned order
release for that item would be: 400> 11 - 0.152 = 471 units.
Planned order release = Planned order receipt 11 - scrap factor2
usIng the MaterIal requIreMents Plan
The people who manage the material requirements planning system are planners. They
are responsible for making detailed decisions that keep the flow of material moving
into, through, and out of the factory. In many companies where there are thousands
of parts to manage, planners are usually organized into logical groupings based on the
similarity of parts or supply.
The basic responsibilities of a planner are to:
1. Launch (release) orders to purchasing or manufacturing.
2. Reschedule due dates of open (existing) orders as required.
3. Reconcile errors and try to find their cause.
4. Solve critical material shortages by expediting or replanning.
5. Coordinate with other planners, master production schedulers, production activity
control, and purchasing to resolve problems.
The material planner works with three types of orders: planned, released, and firm.
Planned orders. Planned orders are automatically scheduled and controlled by
the computer. As gross requirements, projected available inventory, and scheduled
receipts change, the computer recalculates the timing and quantities of planned
order releases. The MRP program recommends to the planner the release of an
order when the order enters the action bucket but does not release the order.
MrP In servIces
MRP systems have essentially been designed and implemented for use in manufacturing environments, where the
number of dependent demand components in the bills of
materials tends to be large and so do the number of calculations. Few service companies face such an environment,
but the fundamental concepts inherent with MRP will still
usually apply.
A cafeteria, such as might be found in many schools,
can serve as an example. Most will use the principle of
MRP, although it is not typically called that. The process
starts with the development of a menu, perhaps on a
monthly or weekly basis. That menu presents the “finished
goods,” in the form of specific completed food offers,
and really is essentially a master schedule. As time draws
nearer to when a food item is to be prepared (the cumulative lead time for making the food and the lead times to
obtain the various ingredients), the cooks need to know the
quantity of food to cook (how many people are expected to
want that food item.) Making too little can draw complaints, while making too much is wasteful as much of it
ends up being thrown out or given away. There is also a
lead time to consider—how long will it take to combine the
ingredients and then cook the item (final assembly). Again,
starting too late will mean the food will not be ready and
starting too early could result in a product that sits too long
and is no longer appealing. That information can be found
on the “bill of material,” which in this case is the recipe
for making the food. That bill of material (recipe) contains
quantities, lead times, and precedent relationships, just as
a bill of material in manufacturing does. The recipe also
usually calls for the steps used to combine the materials,
which is essentially the same as what in manufacturing is
represented by a separate document called the routing.
Once the quantity of each ingredient to make
the right amount of the final food item is known, it is
compared to the existing inventory of the ingredient. The
difference, of course, needs to be obtained. There are
lead times for each of those ingredients, and offsetting
the lead times, just as in MRP, will tell when and how
much of each ingredient needs to be ordered in order that
everything needed to make the final item is present in its
freshest possible form in time to start to actually combine
and make the food item.
The important issue to remember here is that all
companies, from the smallest service company to the
largest manufacturing company, essentially have the same
planning and control issues of long-range planning, master
scheduling, inventory management, capacity planning and
management, controlling production, and quality management. The primary difference lies in the vocabulary (what
they call the activity) and how formally it is done. The
similarities are that effective companies do these things
well, and companies that do not do them well tend to be
much less effective.
Material requirements Planning
93
Released orders. Releasing, or launching, a planned order is the responsibility of the
planner. When released, the order becomes an open order to the factory or to purchasing
and appears on the MRP record as a scheduled receipt. It is then under the control of the
planner, who may expedite, delay, or even cancel the order.
Firm planned orders. The computer-based MRP system automatically recalculates
planned orders as the gross requirements change. At times, the planner may prefer to
hold a planned order firm against changes in quantity and time despite what the computer calculates. This might be necessary because of future availability of material or
capacity or special demands on the system. The planner can tell the computer that the
order is not to be changed unless the planner advises the computer to do so. The order
is “firmed” or frozen against the logic of the computer.
The MRP software nets, offsets, and explodes requirements and creates planned
order releases. It keeps priorities current for all planned orders according to changes in
gross requirements for the part. But it does not issue purchase or manufacturing orders or
reschedule open orders. However, it does print action or exception messages, suggesting
that the planner should act and what kind of action might be appropriate to keep the supply
and demand in balance.
Exception messages If the manufacturing process is under control and the material requirements planning system is working properly, the system will work according
to plan. However, sometimes there are problems that need the attention of the planner.
An MRP system generates exception messages to advise the planner when some event
needs attention. Following are some examples of situations that will generate exception
messages.
Components for which planned orders are in the action bucket and which should be
considered for release.
■■ Open orders for which the timing or quantity of scheduled receipts does not satisfy the
plan. Perhaps a scheduled receipt is timed to arrive too early or late, and its due date
should be revised.
■■ Situations in which the standard lead times will result in late delivery of a zero-level
part. This situation might call for expediting to reduce the standard lead times.
■■
Transaction messages Transaction messages mean that some event has occurred
and must be reflected in the software in order to ensure the MRP records are updated. For
example, when the planner releases an order, or a scheduled receipt is received, or when
any change to the data occurs, that action must be entered into the software. Otherwise,
the records will be inaccurate, and the plan will become unworkable.
Material requirements planners must manage the parts for which they are responsible.
This means not only releasing orders to purchasing and the factory, rescheduling due dates
of open orders, and reconciling differences and inconsistencies but also finding ways to
improve the system and removing the causes of potential error. If the right components are
to be in the right place at the right time, the planner must manage the process.
Managing the Material Requirements Plan
The planner receives feedback from many sources such as
Suppliers’ actions through purchasing.
Changes to open orders in the factory such as early or late completions or differing
quantities.
■■ Management action such as changing the master production schedule.
■■
■■
The planner must evaluate this feedback and take corrective action if necessary. The
planner must consider three important factors in managing the material requirements plan.
Priority Priority refers to maintaining the correct due dates by constantly evaluating
the true due date need for released orders and, if necessary, expediting or de-expediting.
94
Chapter four
Consider the following MRP record. The order quantity is 300 units and the lead time
is 3 weeks.
Week
1
2
3
4
5
Gross Requirements
Scheduled Receipts
Projected Available 150
Net Requirements
Planned Order Receipt
Planned Order Release
100
50
150
200
50
0
100
300
200
50
150
150
300
300
What will happen if the gross requirements in week 2 are changed from 50 to 150 units?
The MRP record will look like the following.
Week
1
2
3
4
5
Gross Requirements
Scheduled Receipts
Projected Available 150
Net Requirements
Planned Order Receipt
Planned Order Release
100
150
150
200
50
−100
100
300
100
250
50
300
50
300
Note that there is a shortage of 100 units in week 2 and that the planned order release
originally in week 2 is now in week 1. What can the planner do? One solution is to expedite the scheduled receipt of 300 units from week 3 to week 2. If this is not possible, the
extra 100 units wanted in week 2 must be rescheduled into week 3. Also, there is now a
planned order release in week 1, and this order should be released.
Bottom-up replanning Action to respond for changed conditions should occur as low
in the product structure as possible. Suppose the part in the previous example is a component
of another part. The first alternative is to expedite the scheduled receipt of 300 into week 2. If
this can be done, there is no need to make any changes to the parent. If the 300 units cannot
be expedited, the planned order release and net requirement of the parent must be changed.
Reducing system nervousness Sometimes requirements change rapidly and by
small amounts, causing the material requirements plan to change back and forth. The planner must judge whether the changes are important enough to react to and whether an order
should be released. One method of reducing system nervousness is firm planned orders.
example Problem
As the MRP planner, you arrive at work Monday morning and look at the MRP record
for part 2876 as shown below.
Order quantity = 30 units
Lead time = 2 weeks
Week
1
2
3
4
5
6
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
35
30
15
10
15
30
15
20
5
20
10
30
5
15
15
30
30
30
20
10
30
30
20
Material requirements Planning
95
usIng Knowledge of the systeM to evaluate ProBleMs
ERP, and even basic MRP, systems are typically difficult
to implement and, if not implemented correctly or not
measured and corrected over time for problems, they can
be the source of an ineffective system with frustrated users. They usually require timely and effective coordination
between functions, accurate and timely recorded data, and
knowledgeable and effective system management.
At times an organization may find that the systems
generate so many exception messages that the people relying on the system information cannot possibly deal with all
the messages during a normal work day. While this can happen on occasion even with a well-implemented and managed
system, when it happens on a routine basis it is a strong
symptom of a system that needs to be “repaired.” Simply
reacting to only “important” messages is a classic example
of “solving the symptom” rather than attacking the core
problem—but with a complex and highly integrated system,
where should one start looking for the core set of problems?
The most logical place to start for most facilities is
the master schedule. There are two reasons for this. First,
in most systems there are significantly fewer master schedules to examine than there are MRP records. Second, for
many products any small change in the master schedule
has the potential to generate hundreds or even thousands
of changes in the MRP records being driven by the master
schedule (occasionally referred to as “system nervousness”). While one would expect to generate some changes
in the master schedule based on changing customer and
internal conditions, they should be considered very carefully for their potential impact before being accepted. If the
master scheduling approach is not managed well, it should
be the first candidate for process improvement. Sometimes
that may require looking at the sales and operations planning (S&OP) approach. For example, did the S&OP provide
for adequate resources of the right type at the right time?
Once it appears the master scheduling process is
being managed well, the next major area to investigate for
problems is data—is data being provided accurately and in a
timely fashion? There are several data systems that need to
be evaluated, including the purchasing area, capacity management, bill of material structures, item master data (lead
time, for example), and production activity control data.
In many cases those two issues—master scheduling and data management—represent the major cause
of exception messages, and proper implementation and
management of those issues should be able to make exception messages just that—exceptions—rather than a routine
expectation.
The computer draws attention to the need to release the planned order for 30 in
week 1. Either you release this order, or there will be a shortage in week 3. During the
first week, the following transactions take place:
a. Only 25 units of the scheduled receipt are received into inventory. The balance is
scrapped.
b. The gross requirement for week 3 is changed to 10.
c. The gross requirement for week 4 is increased to 50.
d. The requirement for week 7 is 15.
e. An inventory count reveals there are 10 more in inventory than the record shows.
f. The 35 gross requirement for week 1 is issued from inventory.
g. The planned order release for 30 in week 1 is released and becomes a scheduled
receipt in week 3.
As these transactions occur during the first week, you must enter these changes
in the computer record. At the beginning of the next week, the MRP record appears
as follows:
Order quantity = 30 units
Lead time = 2 weeks
Week
2
3
4
5
6
7
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
10
50
15
20
15
10
10
30
30
10
20
30
5
20
10
30
30
30
25
5
30
30
20
The opening on-hand balance for week 2 is 20 (20 + 25 + 10 – 35 = 20). The planned
order release originally set in week 4 has shifted to week 3. Another planned order has
96
Chapter four
been created for release in week 5. More importantly, the scheduled receipt in week 3 will
not be needed until week 4. You should reschedule this to week 4. The planned order in
week 2 should be released and become a scheduled receipt in week 4.
suMMary
The job of the MRP is to produce the right components at the right time so that the MPS
can be maintained. The MRP depends on accurate bills of material and on accurate inventory records. Bills of material can be created in many ways, but one department (or individual) must be responsible for them. Inventory records are indispensable to the MRP, and
the MRP is only as good as the inventory records.
The MRP exploding and offsetting processes outlined in this chapter are largely done
by the computer. The logic used is repetitive and, while error prone when done by individuals, can be accomplished well by computer. Good MRP practice is achieved by planners
being able to work with the system.
The MRP process uses the bill of materials that lists components used to make a product, the lead time to make or obtain those components, and the existing inventory of those
components to calculate a series of planned order releases to obtain or make components
to meet future product needs.
Key terMs
Action bucket 87
Allocate 86
Bill of material 74
Bottom-up replanning 94
Bucketless system 87
Component items 75
Dependent demand 72
Exception messages 93
Exploding 82
Firm planned orders 93
Independent demand 72
Indented bill 79
Inventory record 74
Lead time 81
Low-level code 88
Master production schedule 74
Material requirements planning
(MRP) 72
Multilevel bills 76
Multiple bill 76
Net requirements 86
Offsetting 82
Open orders 86
Parent 75
Pegging report 80
Planned orders 92
Planned order receipt 82
Planned order release 82
Planning bills 79
Planning factors 74
Planning horizon 87
Product tree 75
Reducing system nervousness 94
Releasing an order 84
Scheduled receipts 86
Single-level bill 76
Summarized parts list 79
Time buckets 86
Transaction messages 93
Where-used report 79
questIons
1. What is a material requirements plan?
2. What is the difference between dependent and independent demand?
3. Should an MRP be used with dependent or independent demand items?
4. What are the objectives of the MRP?
Material requirements Planning
97
5. What is the relationship between the MPS and the MRP?
6. Why is a computer necessary in an MRP system?
7. What are the major inputs to the MRP system?
8. What data is found in a part master file or an item master file?
9. What is a bill of material? What are two important points about bills of material?
10. To what does bill of material structure refer? Why is it important?
11. Describe the parent–component relationship.
12. Describe the following types of bills of material:
a. Product tree.
b. Multilevel bill.
c. Single-level bill.
d. Indented bill.
e. Summarized parts list.
f. Planning bill.
13. Why do MRP computer programs store single-level bills?
14. Describe each of the seven uses of a bill of material described in the text.
15. What are where-used and pegging reports? Give some of their uses.
16. Describe the processes of offsetting and exploding.
17. What is a planned order? How is it created?
18. From where does the gross requirement of a component come?
19. Who is responsible for releasing an order? Describe what happens to the inventory records and
to PAC and purchasing.
20. What is a scheduled receipt? From where does it originate?
21. What is an open order? How does it get closed?
22. What is the meaning of the term low-level code? How is the low-level code of an MPS part
represented?
23. What are the responsibilities of a material requirements planner?
24. Give two examples of processes with inherent scrap. Hint, the use of natural products often
involves some scrap.
25. What would make the planned order release for an item different from the planned order receipt?
26. Describe the differences among planned orders, released orders, and firm planned orders. Who
controls each?
27. What are exception messages? What is their purpose?
28. What is a transaction message? Why is it important?
29. What are the three important factors in managing the material requirements plan? Why is each
important?
30. Describe the problems that might come from using an incorrect bill of material in MRP.
31. Describe how MRP might be used to plan for a change in design for a product.
ProBleMs
4.1. Using the following product tree, construct the appropriate single-level trees. How
many Cs are needed to make 50 Xs and 100 Ys?
X
A(2)
C(2)
Answer.
D
400 Cs
Y
B
B
E(2)
E(2)
C(2)
98
Chapter four
4.2. Given the following parents and components, construct a product tree. Figures in
parentheses show the quantities per item. How many Gs are needed to make one A?
Parent
Component
A
B(2)
C(4)
D(4)
B
E(2)
F(1)
C
G(3)
E
G(4)
F(3)
H(2)
4.3. Using the following product tree, determine the planned order receipts and planned
order releases if 200 As are to be produced in week 5. All lead times are 1 week
except for component E, which has a lead time of 2 weeks.
A
B
C
D
E
Week
1
Part A
Lead Time: 1 week
Planned Order Receipt
Planned Order Release
Part B
Lead Time: 1 week
Planned Order Receipt
Planned Order Release
Part C
Lead Time: 1 week
Planned Order Receipt
Planned Order Release
Part D
Lead Time: 1 week
Planned Order Receipt
Planned Order Release
Part E
Lead Time: 2 weeks
Planned Order Receipt
Planned Order Release
2
3
4
5
4.4. Complete the following table. Lead time for the part is 2 weeks, and the order quantity is 50. What action should be taken?
Week
1
2
3
4
Gross Requirements
40
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
20
15
15
20
Answer.
An order for 50 should be released in week 1.
4.5. Given the following product tree, explode, offset, and determine the gross and net
requirements. All lead times are 1 week, and the quantities required are shown in
parentheses. The master production schedule calls for 100 As to be available in week 5.
There are 20 Bs available. All other on-hand balances = 0.
A
B(2)
D(1)
C(1)
E(1)
D(2)
F(1)
Material requirements Planning
1
Week
Part A
3
4
5
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Lead Time: 1 week
Part B
Lead Time: 1 week
Part C
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Lead Time: 1 week
Part D
Lead Time: 1 week
Part E
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Lead Time: 1 week
Part F
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Lead Time: 1 week
Answer.
2
99
Planned order releases are
Part A: 100 in week 4
Part B: 180 in week 3
Part C: 100 in week 3
Part D: 380 in week 2
Part E: 180 in week 2
Part F: 100 in week 2
4.6. Complete the following table. Lead time for the part is 2 weeks. The lot size is 100. What
is the projected available at the end of week 3? When is it planned to release an order?
Week
1
2
3
4
Gross Requirements
Scheduled Receipts
30
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
20
65
100
35
25
Answer. Projected available at the end of week 3 is 10.
An order release is planned for the beginning of week 2.
100
Chapter four
4.7. Complete the following table. Lead time for the part is 2 weeks. The lot size is 50. What
is the projected available at the end of week 3? When is it planned to release an order?
Week
1
2
3
4
Gross Requirements
Scheduled Receipts
10
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
30
50
25
10
10
4.8. Given the following partial product tree, explode, offset, and determine the gross and
net requirements for components H, I, J, and K. There are other components, but they
are not connected to this problem. The quantities required are shown in parentheses.
The master production schedule calls for the completion of 60 Hs in week 3 and 80 in
week 5. There is a scheduled receipt of 120 Is in week 2. There are 400 Js and 400 Ks
available. All lot sizes are lot-for-lot.
H
I(2)
J(2)
Week
Part H
Lead Time: 1 week
Part I
Lead Time: 2 weeks
Part J
Lead Time: 1 week
Part K
Lead Time: 1 week
Answer.
K(3)
1
2
3
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available 400
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available 400
Net Requirements
Planned Order Receipt
Planned Order Release
There is a planned order release for part K of 80 in week 1.
4
5
Material requirements Planning
101
4.9. MPS parent X has planned order releases of 30 in weeks 2 and 4. Given the following product tree, complete the MRP records for parts Y and Z. Quantities required are
shown in brackets.
X
Y(1)
S
Z(2)
T
Week
Part Y
Lead Time: 2 weeks
Lot Size: 50
1
2
3
4
3
4
Gross Requirements
Scheduled Receipts
Projected Available
30
Net Requirements
Planned Order Receipt
Planned Order Release
Week
Part Z
Lead Time: 1 week
Lot Size: 100
1
2
Gross Requirements
Scheduled Receipts
Projected Available
20
Net Requirements
Planned Order Receipt
Planned Order Release
4.10. Given the following product tree, explode, offset, and determine the gross and net
requirements. The quantities required are shown in parentheses. The master production schedule calls for the completion of 100 As in week 5. There is a scheduled
receipt of 100 Bs in week 1. There are 200 Fs available. All order quantities are
lot-for-lot.
A
B(2)
D(1)
C(1)
E(1)
D(2)
F(1)
102
Chapter four
1
Week
Part A
Lead Time: 1 week
Part B
Lead Time: 1 week
Part C
Lead Time: 1 week
Part D
Lead Time: 1 week
Part E
Lead Time: 1 week
Part F
Lead Time: 1 week
2
3
4
5
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
4.11. Given the following product tree, complete the MRP records for parts X, Y, W, and
Z. Note that parts X and Y have specified order quantities.
X
Y(3)
Z(1)
Z(2)
W(1)
Material requirements Planning
Part X
Lead Time: 1 week
Lot Size: 20
Part Y
Lead Time: 2 weeks
Lot Size: 50
Part Z
Lead Time: 1 week
Lot Size: lot-for-lot
Part W
Lead Time: 1 week
Lot Size: 400
103
Week
1
2
3
4
5
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
15
20
10
15
10
15
10
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
50
30
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
90
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
4.12. Given the following product tree, determine the low-level codes for all the components.
F
A
C
E
D
D
E
B
D
Item
A
B
B
C
E
D
F
Low-Level Code
4.13. Given the following product tree, determine the low-level codes for all the components.
B
A
D
C
G
J
Item
A
Low-Level Code
C
H
H
H
E
G
H
B
C
D
E
F
F
E
J
C
G
H
J
104
Chapter four
4.14. Given the following product tree, develop a material requirements plan for the components. Quantities per are shown in parentheses. The following worksheet shows
the present active orders, the available balances, and the lead times.
A
B
F
C(2)
Low-Level Code
C
C
D(2)
Week
Part A
0
Lead Time: 1 week
Lot-for-lot
Part F
0
Lead Time: 1 week
Lot-for-lot
Part B
Lead Time: 2 weeks
Lot Size: 300
Part C
Lead Time: 2 weeks
Lot Size: Lot-forlot
Part D
Lead Time: 2 weeks
Lot Size: 300
Part E
Lead Time: 3 weeks
Lot Size: 500
D
1
E
2
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
3
4
60
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
5
70
100
Gross Requirements
Scheduled Receipts
Projected Available 200
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
120
300
Gross Requirements
Scheduled Receipts
Projected Available 400
Net Requirements
Planned Order Receipt
Planned Order Release
Answer. The low-level code for part D is 2. There is a planned order release of
300 for part D in week 1. There are no planned order releases for part E. There is a
planned order release of 100 for Part C in week 1 and 140 in week 2.
4.15. Given the following product tree, explode, offset, and determine the gross and net
requirements. All lead times are 1 week, and the quantities required are shown in
parentheses. The master production schedule calls for the completion of 100 As in
week 4 and 50 in week 5. There are 300 Bs scheduled to be received in week 1 and
200 Ds in week 3. There are also 20 As available.
Material requirements Planning
105
A
B(2)
D(1)
C(1)
E(1)
D(2)
B(1)
D(1)
Low-Level
Code
E(1)
Week
Part A
1
2
3
4
5
Gross Requirements
Scheduled Receipts
Projected Available 20
Net Requirements
Planned Order Receipt
Planned Order Release
Lead Time: 1 week
Lot Size: lot-for-lot
Part B
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Lead Time: 1 week
Lot Size: lot-for-lot
Part C
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Lead Time: 1 week
Lot Size: lot-for-lot
Part D
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Lead Time: 1 week
Lot Size: lot-for-lot
Part E
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Lead Time: 1 week
Lot Size: lot-for-lot
4.16. Given the following product tree, determine the low-level codes and the gross and
net quantities for each part. There is a requirement for 100 As in week 4 and 50 Bs
in week 5. There is a scheduled receipt of 100 Cs in week 2. Quantities required of
each are also shown.
A
B
C(2)
E(1)
D(1)
F(1)
E(2)
E(1)
C(1)
E(1)
C(1)
E(1)
F(1)
F(1)
106
Chapter four
Low-Level
Code
Week
Part A
Lead Time: 1 week
Lot Size: lot-for-lot
Part B
Lead Time: 1 week
Lot Size: lot-for-lot
Part C
Lead Time: 1 week
Lot Size: lot-for-lot
Part D
Lead Time: 1 week
Lot Size: lot-for-lot
Part E
Lead Time: 1 week
Lot Size: 500
Part F
Lead Time: 1 week
Lot Size: lot-for-lot
1
2
3
4
5
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
4.17. Complete the following MRP record. The lead time is 4 weeks, and the lot size is
200. What will happen if the gross requirements in week 3 are increased to 150
units? As a planner, what actions can you take?
Initial MRP
Week
1
2
3
4
5
Gross Requirements
Scheduled Receipts
Projected Available 100
Net Requirements
Planned Order Receipt
Planned Order Release
50
125
200
100
60
200
40
1
2
3
4
5
Revised MRP
Week
Gross Requirements
Scheduled Receipts
Projected Available 100
Net Requirements
Planned Order Receipt
Planned Order Release
Material requirements Planning
107
4.18. It is Monday morning, and you have just arrived at work. Complete the following
MRP record as it would appear Monday morning. Lead time is 2 weeks, and the lot
size is 100.
Initial MRP
Week
1
2
3
4
5
Gross Requirements
Scheduled Receipts
50
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
70
100
40
80
50
40
During the week, the following events occur. Enter them in the MRP record.
a. The planned order for 100 in week 1 is released.
b. Thirty of the scheduled receipts for week 1 are scrapped.
c. An order for 30 is received for delivery in week 3.
d. An order for 50 is received for delivery in week 6.
e. The gross requirements of 70 in week 1 are issued.
MRP record at the end of week 1
Week
2
3
4
5
6
Gross Requirements
Scheduled Receipts
Projected Available
Net Requirements
Planned Order Receipt
Planned Order Release
4.19. The following bill of materials represents the major components for a computer
system.
Acme 800 Deluxe
Computer System
Speakers
(2 needed)
Installation
Kit
Cable
Kit
Monitor
Software
Kit
CD-ROM
Drive
System Unit
Hard
Disk
Keyboard
Packaging
Processor
Complete the MRP records below. Note the following:
Production plans (the MPS) for the 800 Deluxe computer system are as follows:
Start assembling 2500 in week 2
Start assembling 3000 in weeks 3, 4, and 5
Start assembling 2000 in week 6
■■ The gross requirements for the system unit have already been given to you. For the
remaining items, you will need to figure out the gross requirements.
■■ All scheduled receipts, lead times, and beginning inventory levels are shown.
■■
108
Chapter four
System Unit
Lead time = 1 week
Minimum order quantity = 500
Week
1
Gross Requirements
2
3
4
5
6
2500
3000
3000
3000
2000
2
3
4
5
6
2
3
4
5
6
Scheduled Receipts
Projected Available
0
Net Requirements
Planned Receipts
Planned Order Releases
Speakers
Lead time = 1 week
Minimum order quantity = 5000
Week
1
Gross Requirements
Scheduled Receipts
5000
Projected Available
0
Net Requirements
Planned Receipts
Planned Order Releases
CD-ROM Drives
Lead time = 4 weeks
Minimum order quantity = 5000
Week
1
Gross Requirements
Scheduled Receipts
Projected Available
11,500
Net Requirements
Planned Receipts
Planned Order Releases
case study 4.1
Apix Polybob Company
Ken Mack, plant manager for the Apix Polybob Company, was having a heated discussion
with Jack Gould, the production and inventory control manager. Ken was getting tired of
frantic calls from Jim Uphouse, the marketing manager, concerning late orders for their
Polybob (polybobs are a fictitious product) customers and was once again after Jack to
solve the problem. Some of the discussion points follow:
Material requirements Planning
109
jack: “Look, Ken, I’m not sure what more we can do. I’ve reexamined the EOQ (economic order quantity lot size) values and all the reorder points for all our inventory for all our Polybob models, including all component levels and purchased
items. I’ve implemented strict inventory control procedures to ensure our accuracy levels to at least 80%, and I’ve worked with the production people to make
sure we are maximizing both labor efficiency and utilization of our equipment.
The real problem is with those salespeople. We no sooner have a production run
nicely going, and they change the order or add a new one. If they’d only leave
us alone for a while and let us catch up with our current late order bank, we’d
be okay. As it is, everyone is getting tired of order changes, expediting, and
making everything into a crisis. Even our suppliers are losing patience with us.
They tend to disbelieve any order we give them until we call them up for a crisis
shipment.”
ken: “I find it hard to believe that you really have the EOQ and reorder point values right. If they were, we shouldn’t have all these part shortages all the time
while our overall inventory is going up in value. I also don’t see any way
we can shut off the orders coming in. I can imagine the explosion from Jim
if I even suggested such a thing. He’ll certainly remind me that our mission
statement clearly points out that our number-one priority is customer service,
and refusing orders and order changes certainly doesn’t fit as good customer
service.”
jack: “Then maybe the approach is to deal with Frank Adams (the chief financial
officer). He’s the one who is always screaming that we have too much inventory, too much expediting cost, too much premium freight costs from suppliers,
and poor efficiency. I’ve tried to have him authorize more overtime to relieve
some of the late order conditions, but all he’ll say is that we must be making
the wrong models. He continually points to the fact that the production hours
we are paying for currently are more than enough to make our orders shipped
at standard, and that condition has held for over a year. He just won’t budge on
that point. Maybe you can convince him.”
ken: “I’m not sure that’s the answer either. I think he has a point, and he certainly
has the numbers to back him up. I’d have a real rough time explaining what
we were doing to Ron Marrison (the CEO). There’s got to be a better answer.
I’ve heard about a systems approach called material requirements planning
or something like that. Why don’t you look into that? Take a representative
model and see if that approach could help us deal with what appears to be an
impossible situation. I’m sure something would work. I know other factories
have similar production conditions yet don’t seem to have all our problems.”
A
B
F
C (2 each)
E (3 each)
F (2 each)
E
D
Following is the information about Polybob model A that Ken suggested as a representative model to use for the analysis:
Component
B
C
D
E
F
Lot
Size
80
150
200
400
500
Inventory
10
40
180
300
50
Lead
Time
1
1
2
2
2
Scheduled
Receipts
None
None
None
None
500, week 1
Reorder
Point
5
15
50
70
80
110
Chapter four
The following are the master schedule production lots for Model A:
Complete 50 units, week 3
Complete 50 units, week 5
Complete 60 units, week 7
Complete 60 units, week 9
Complete 50 units, week 11
Upon seeing this information, Jack stated, “Look at how regular our production
schedule is for this model. The reorder points will more than cover requirements, and none
have lead times that make it tough to respond. This analysis should show that all the work
I did on EOQ and reorder points was right, and the real problem lies with those sales and
finance people who don’t understand our production needs.”
assignment
1. What are the key issues brought about in the conversation? What are the key symptoms, and what are the underlying problems? Be specific in your answers.
2. Use the product information to develop an MRP approach to the problems. Would
MRP solve the problems? If so, show specifically how MRP would avoid the problems
discussed by Ken and Jack.
3. Do any conditions bother you about the ability of MRP to deal with the problems?
What specifically are those conditions?
4. Suppose it was discovered that only 250 of component E were in stock instead of the
300 listed on the inventory record. What problems would this cause (if any), and what
are some of the ways that these problems could be addressed? How would (if at all)
MRP help you when other methods might not?
5. Suppose that the design engineer advises that he has a new design for component F.
It won’t be ready until sometime after week 2, but he wants you to give a date for the
first supplier shipment to come in, and you should be ready to tell the supplier how
many to ship. Since the change is transparent to the customer, the design engineer
advises you to go ahead and use up any existing material of the model. How will
MRP help you to deal with this issue?
6. Can you think of any other “what if” questions that might be more easily addressed
by a systematic approach such as MRP?
case study 4.2
Benzie Products Company
Benzie Products Company produces several lines of products, but one (they call it “product X”) uses unique parts to produce it and the demand is very seasonal. There are some
possible variations in the design, so the company tends to use available-to-promise (ATP)
logic to master schedule the product. Since the components to produce it are quite expensive, the company tries very hard to minimize any inventory of the product or its components during the seasons with very low sales. Product X is just now entering the low
season, and the following chart represents the forecast data and actual customer orders for
the next ten weeks:
Week
Forecast
Customer Orders
Master Production Schedule
1
25
27
2
20
21
50
3
16
16
4
16
13
5
15
11
50
6
15
9
7
13
7
8
11
4
50
9
10
3
10
9
2
Material requirements Planning
111
There are currently (at the start of week 1) 27 product X left in inventory. The following represents the product structure for product X:
X
A
C
B(2)
D
C(2)
The following table gives the relevant data for components A, B, C, and D at the start
of week 1:
Component
Starting Inventory
Lead Time (weeks)
Lot Size
Safety Stock
A
0
2
60
0
B
2
1
100
0
C
D
212 63
2
4
250 100
0
0
In addition, component A has a scheduled receipt of 60 units for week 2.
assignment
1. Complete the master schedule for Product X, including the projected available inventory and the ATP numbers.
2. From the master schedule for Product X and using the data given for components A,
B, C, and D, create MRP grids for each of the components for the next ten weeks.
3. Suppose that a customer for Product X wants three additional units for their order
scheduled for week 4. What would you tell them? Specifically, if you cannot promise
them the three units for week 4, what is the best that you can do given the information
you have. Assume that adequate capacity exists in all the production equipment.
4. The customer described in question 3 decides against placing the order for the extra
units in week 4, but shortly thereafter you are informed that someone in the warehouse dropped a box of component C and broke 20 of them. They had to be scrapped.
Describe the consequences and a plan of action to deal with the problem. Assume the
lead time for component C is a firm 2 weeks.
5. It is clear that the company has set the safety stock (planning for extra material “just
in case” something goes wrong) level for components at zero in order to minimize
their inventory during the slow season. Discuss this policy, pointing out the pros and
cons of such a policy. Develop what you might suggest as a policy for safety stock
given the information available in the case.
Chapter
five
CapaCity ManageMent
introDuCtion
So far we have been concerned with planning priority, that is, determining what is to be produced and when. The system is hierarchical, moving from long planning horizons and few
details (production plan) through medium time spans (master production schedule) to a high
level of detail and short time spans (material requirements plan). At each level, manufacturing develops priority plans to satisfy demand. However, without the resources to achieve the
priority plan, the plan will be unworkable. Capacity management is concerned with supplying
the necessary resources. This chapter looks more closely at the question of capacity: what it
is, how much is available, how much is required, and how to balance priority and capacity.
Definition of CapaCity
Capacity is the amount of work that can be done in a specified time period. APICS
Dictionary, 14th edition defines capacity as “the capability of a worker, machine, work
center, plant, or organization to produce output per time period.” Capacity is a rate of doing
work, not the quantity of work done. Two kinds of capacity are important: the capacity available and the capacity required. Capacity available is the capacity of a system or resource
to produce a quantity of output in a given time period. Capacity required is the capacity
of a system or resource needed to produce a desired output in a given time period. A term
closely related to capacity required is load. This is the amount of released and planned work
assigned to a facility for a particular time period. It is the sum of all the required capacities.
These three terms—capacity required, load, and capacity available—are important in capacity management and will be discussed in subsequent sections of this chapter.
Capacity is often pictured as a funnel, as shown in Figure 5.1. Capacity available is
the rate at which work can be withdrawn from the funnel. Load is the amount of work in
the funnel.
Capacity management is responsible for determining the capacity needed to achieve
the priority plans as well as providing, monitoring, and controlling that capacity so the
LOAD
CAPACITY
AVAILABLE
OUTPUT
Figure 5.1 Capacity versus load.
112
Capacity Management
113
priority plan can be met. APICS Dictionary, 14th edition defines capacity management
as “the function of establishing, measuring, monitoring, and adjusting limits or levels of
capacity in order to execute all manufacturing schedules.” As with all management processes, it consists of planning and control functions.
Capacity planning is the process of determining the resources required to meet the
priority plan and the methods needed to make that capacity available. It takes place at each
level of the priority planning process. Production planning, master production scheduling,
and material requirements planning determine priorities, i.e., what is wanted and when.
These priority plans cannot be implemented, however, unless the firm has sufficient
capacity to fulfill the demand. Capacity planning links the various production priority
schedules to manufacturing resources.
Capacity control is the process of monitoring production output, comparing it with
capacity plans, and taking corrective action when needed. Capacity control will be examined further in Chapter 6.
CapaCity planning
Capacity planning involves calculating the capacity needed to achieve the priority plan
and finding ways of making that capacity available. If the capacity requirement cannot be
met, the priority plans are unachievable and have to be changed.
Priority plans are usually stated in units of product or some standard unit of output.
Capacity can sometimes be stated in the same units, such as tons of steel or yards of cloth.
If there is no common unit, capacity is stated as the hours available. The priority plan must
then be translated into hours of work required and compared to the hours available. The
process of capacity planning is as follows:
1. Determine the capacity available at each work center in each time period.
2. Determine the load at each work center in each time period.
Translate the priority plan into the hours of work required at each work center in
each time period.
■■ Sum up the capacities required for each item on each work center to determine the
load on each work center in each time period.
3. Resolve differences between available capacity and required capacity. If possible,
adjust available capacity to match the load. Otherwise, the priority plans must be
changed to match the available capacity.
■■
This process occurs at each level in the priority planning process, varying only in the
level of detail and time spans involved.
Planning Levels
Resource planning involves long-range capacity resource requirements and is directly
linked to production planning. Typically, it involves translating monthly, quarterly, or
annual product priorities from the production plan into some total measure of capacity, such as gross labor hours. Resource planning involves changes in staffing, capital
equipment, product design, or other facility changes that take a long time to acquire and
eliminate. If a resource plan cannot be devised to meet the production plan, the production
plan has to be changed. The two plans set the limits and levels for production. If they are
realistic, the master production schedule (MPS) should be achievable. (See the Resource
Planning section in Chapter 2, Production Planning System.)
Rough-cut capacity planning (RCCP) takes capacity planning to the next level
of detail. The master production schedule is the primary information source for RCCP.
The purpose of rough-cut capacity planning is to check the feasibility of the MPS, provide warnings of any bottlenecks, ensure utilization of work centers, and advise vendors
of capacity requirements. (See the Rough-Cut Capacity Planning section of Chapter 3,
Master Scheduling.)
114
Chapter five
Priority
Capacity
PRODUCTION
PLAN
RESOURCE
PLAN
Long Range
MASTER
PRODUCTION
SCHEDULE
PLAN
ROUGH-CUT
CAPACITY
PLAN
Medium Range
MATERIAL
REQUIREMENTS
PLAN
CAPACITY
REQUIREMENTS
PLAN
Short Range
IMPLEMENT/
CONTROL
PRODUCTION
ACTIVITY
CONTROL
CAPACITY
CONTROL
Short Range
Figure 5.2 Planning levels.
Capacity requirements planning (CRP) is capacity planning at a more detailed
level and is directly linked to the material requirements plan. Since this type of planning
focuses on component parts, greater detail is involved than in rough-cut capacity planning.
It is concerned with individual orders at individual work centers and calculates work center loads and labor requirements for each time period at each work center.
Figure 5.2 shows the relationship between the different levels of priority planning and
capacity planning. Notice that, although the upper levels of priority planning are input to
lower levels, the various capacity plans relate only to their level in the priority plan, not to
subsequent capacity planning levels. Resource planning relates to production planning but
is not an input to rough-cut capacity planning.
After the plans are completed, production activity control and purchasing must be
authorized to process shop orders and purchase orders. Capacity must still be considered
at this lowest level of detail. Work center capacity control will be covered in Chapter 6.
CapaCity requireMents planning
The capacity requirements plan (CRP) occurs at the level of the material requirements
plan (MRP). It is the process of determining in detail the amount of labor and machine
resources needed to achieve the required production. Planned orders from the MRP and
open shop orders (scheduled receipts) are converted into demand for time in each work
center in each time period. This process takes into consideration the lead times for operations and offsets the operations at work centers accordingly. In considering open shop
orders, it accounts for work already done on a shop order. Capacity requirements planning
is the most detailed, complete, and accurate of the capacity planning techniques. This
accuracy is most important in the immediate time periods. Because of the detail, a great
amount of data and computation is required.
Inputs
The inputs needed for CRP are open shop orders, planned order releases, routings, time
standards, lead times, and work center capacities. This information can be obtained from
the following data:
■■
■■
Open orders.
Material requirements plan.
Capacity Management
■■
■■
115
Routings.
Work centers.
Open orders An open shop order appears as a scheduled receipt on the material
requirements plan. It is a released order for a quantity of a part to be manufactured and
completed on a specific date. It shows all relevant information, such as quantities, due
dates, and operations. A record of all the active shop orders is maintained manually or as
a computer file or table.
Planned order releases Planned orders are determined by the computer’s MRP
logic based upon the gross requirements for a particular part. They are inputs to the CRP
process in assessing the total capacity required in future time periods.
Routings A routing is the path that work follows from work center to work center as it
is completed. A routing is specified on a route sheet or in a computer routing file or table.
Routing data should exist for every component manufactured and contain the following
information:
Operations to be performed.
Sequence of operations.
■■ Work centers to be used.
■■ Possible alternate work centers.
■■ Tooling needed at each operation.
■■ Standard times: setup times and run times per piece.
■■
■■
Figure 5.3 shows an example of routing information.
Work centers A work center is comprised of a number of machines or workers
capable of doing the same work. The equipment will normally be similar so there are no
differences in the kind of work the machines can do or the capacity of each. Several sewing
machines of similar capacity could be considered a work center. Work center data contains
information on the capacity and move, wait, and queue times associated with the center.
The move time is the time normally taken to move material from one workstation to
another. The wait time is the time a job is at a work center after completion and before
being moved. The queue time is the time a job waits at a work center before being processed. Lead time is the sum of queue, setup, run, wait, and move times.
Shop calendar Another piece of information needed is the number of working days
available at a work center The Gregorian calendar, which is the one used in daily activities, has some serious drawbacks for manufacturing planning and control. The months do
not have the same number of days, holidays are spread unevenly throughout the year, and
the calendar does not work on a decimal base. Suppose that the lead time for an item is
35 working days and on December 13 an order is placed for delivery by January 22. This
is about six weeks away, but with the Gregorian calendar, some calculations have to be
Part Name: Gear shaft
Drawing Number: D123X
Part Number: SG 123
Operation
No.
Work
Center
S/U Time
(standard hours)
Run Time/Piece
(standard hours)
10
20
30
40
50
12
14
17
03
Stores
1.50
0.50
0.30
0.45
0.20
0.25
0.05
0.10
Figure 5.3 Routing file.
Operation
Turn shaft
Mill slot
Drill 2 holes
Grind
Inventory
116
Chapter five
MONTH
WEEK
27
WED.
THURS.
2
3
4
5
9
124
10
127
16
29
23
18
24
30
142
2
123
130
134
138
135
139
1
143
WORK DAY
144
140
123
8
14
15
21
22
28
29
4
5
141
3
145
7
136
27
2
SUN.
131
20
26
SAT.
126
13
19
25
31
6
12
129
133
137
31
11
17
FRI.
125
128
132
30
JULY 2
TUES.
123
28
JULY
MON.
146
DEFINES NON-WORK DAYS
Figure 5.4 Planning calendar. (Source: The American Production and Inventory Control
Society, Inc., Material Requirements Planning Training Aid, 5-21. Reprinted with
permission.)
made to decide if there is enough time to make the delivery. Holidays occur in that period,
and the plant will be shut down for inventory the first week in January. How many working days really exist?
Because of these problems, it is desirable to develop a shop calendar. This can be set
up in different ways, but the example shown in Figure 5.4 is typical.
CapaCity available
We’ve already discussed that capacity available is the capacity of a system or resource to
produce a quantity of output in a given time period. It is affected by the following:
Product specifications. If the product specifications change, the process and time
required to make the product may change, thus affecting the number of units that can
be produced in a given time period.
■■ Product mix. Each product has its own set of processes and time requirements in what
it takes to make the product. If the mix of products being produced changes, the total
time, or capacity required, for the mix may change.
■■ Plant and equipment. This relates to the methods used to make the product. If the
method is changed—for example, a faster machine is used—the rate of output may
change. Similarly, if more machines are added to the work center, the capacity available will change.
■■ Work effort. This relates to the speed or pace at which the work is done. If the workforce changes pace, perhaps producing more in a given time, the capacity will be
altered.
■■
All of these elements have an impact on capacity. If these vary considerably, it is difficult to use units of product to measure capacity. So what units should be used to measure
capacity?
Measuring Capacity
Units of output If the variety of products produced at a work center or in a plant is
not large, it is often possible to use a unit common to all products. Paper mills measure
capacity in tons of paper, breweries in barrels of beer, and automobile manufacturers in
Capacity Management
117
numbers of cars. However, if a variety of products is made, a good common unit may not
exist. In this case, the unit common to all products is time.
Standard time The work required to make a product is expressed as the time required
to make the product using a given method of manufacture. Using time study techniques,
the standard time (also known as standard hours) for a job can be determined, that is,
the time it would take an average qualified operator working at a normal pace to do the
job. It provides a yardstick for measuring work and a unit for stating capacity. It is also
used in loading and scheduling work to be done.
Levels of Capacity
Capacity is usually measured on at least three levels:
Machine or individual worker.
Work center, production line or cell.
■■ Plant, which can be considered as a group of multiple work centers.
■■
■■
Determining Capacity Available
The factors of available time, utilization, and efficiency are used to determine the three
types of capacity available: theoretical; calculated or rated; and demonstrated or measured.
Available time The available time is the number of hours a work center can be used.
For example, a work center working one 8-hour shift for 5 days a week is available 40
hours a week. The available time depends on the number of machines, the number of
workers, and the hours of operation.
example Problem
A work center has 3 machines and is operated for 8 hours a day, 5 days a week. What
is the available time?
Answer
Available time = 3 * 8 * 5 = 120 hours per week
Utilization The available time is the maximum hours one can expect from the work
center. However, it is unlikely this will be attained all the time. Downtime can occur due
to machine breakdown, absenteeism, training, lack of material, and various problems that
cause unavoidable delays or idle time. If the machine running is also dependent on an individual, time taken for lunch or breaks must also be considered. The percentage of time that
the work center is active compared to the available time is called work center utilization:
Utilization =
hours actually worked
* 100%
available hours
example Problem
A work center is available 120 hours but actually produces goods for only 100 hours.
What is the utilization of the work center?
Answer
Utilization =
100
* 100% = 83.3%
120
Utilization can be determined from historical records or by a work sampling study.
Efficiency It is possible for a work center to utilize 100 hours a week but not produce
100 standard hours of work. Efficiency measures the output as compared to the standard.
The workers might be working at a faster or slower pace than the standard working pace,
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Chapter five
causing the efficiency of the work center to be more or less than 100%. For example, in a
given shift, the expected output may be 100 hours, or the production of 50 units. But the
actual output is 120 hours, which produced 60 units.
Efficiency =
standard hours produced
* 100%
hours actually worked
example problem
A work center produces 120 standard hours of work in 100 hours. What is the efficiency?
Efficiency =
120
* 100% = 120%
100
Productivity APICS Dictionary, 14th edition defines productivity as “the overall
measure of the ability to produce a good or a service.” It compares the actual output of
production to the actual input of all resources, incorporating the utilization of the time
available and the efficiency during that time. There are many ways of measuring productivity. An example of a productivity calculation that can be applied to each work center
uses the following formula:
Productivity = utilization * efficiency * 100%
Example: A work center operates 32 hours of the 40 hours available. During that time,
they produce output equivalent to 30 hours of work.
32
30
*
* 100% = 75%
Productivity =
40
32
Theoretical capacity is the maximum capacity available, with no regard for downtime, utilization, or efficiency. If a company uses two 8-hour shifts at a work center, the
theoretical daily capacity would be 16 hours.
Rated capacity The available capacity at a work center for a period of time, and
accounting for the average utilization and efficiency of that work center, is known as
calculated or rated capacity.
Rated capacity = available time * utilization * efficiency
example Problem
A work center consists of 4 machines and is operated 8 hours per day for 5 days a
week. Historically, the utilization has been 85% and the efficiency 110%. What is the
rated capacity?
Answer
Available time = 4 * 8 * 5 = 160 hours per week
Rated capacity = 160 * 0.85 * 1.10 = 149.6 standard hours
The expectation is to produce 149.6 standard hours of work from that work center in
an average week.
Demonstrated Capacity
The historical output, or capacity, of a work center, is known as demonstrated, or measured,
capacity. This type of capacity examines the previous production records and uses that information as the available capacity of the work center. This is determined in part by the actual
load input to the work center, and is not necessarily reflective of what the work center is
capable of producing.
example Problem
Over the previous 4 weeks, a work center produced 120, 130, 150, and 140 standard
hours of work. What is the demonstrated capacity of the work center?
Capacity Management
119
Answer
Demonstrated capacity =
120 + 130 + 150 + 140
= 135 standard hours
4
Notice that demonstrated capacity is average, not maximum, output. It also depends
on the utilization and efficiency of the work center, although these are not included in the
calculation, as they would already have been taken into account in the production records
used in the calculation.
Efficiency and utilization can be obtained from historical data if a record is maintained of the hours available, hours actually worked, and the standard hours produced by
a work center.
example Problem
Over a 4-week period, a work center produced 540 standard hours of work, was available for work 640 hours, and actually worked 480 hours. Calculate the utilization and
the efficiency of the work center.
Answer
Utilization =
hours actually worked
480
* 100 =
* 100% = 75%
available hours
640
Efficiency =
standard hours of work produced
540
* 100 =
* 100% = 112.5%
hours actually worked
480
Safety capacity One other type of capacity that is often used by companies is known
as safety capacity. This capacity is available capacity that is planned to exceed capacity
required. It is used to protect against unplanned activities, such as breakdowns, poor quality, preventive maintenance, and so forth. It is also referred to as a capacity cushion, and
can be used as an alternative to safety stock. Care should be taken in using both safety
capacity and safety stock, as this can cause additional costs for the firm.
CapaCity requireD (loaD)
Capacity requirements are generated by the priority planning system and involve translating priorities, given in units of product or some common unit, into hours of work required
at each work center in each time period. As mentioned previously, this translation takes
place at each of the priority planning levels, from production planning to master production scheduling to material requirements planning. Figure 5.2 illustrates this relationship.
The level of detail, the planning horizon, and the techniques used vary with each
planning level. In this chapter, the material requirements planning/capacity requirements
planning level will be discussed.
Determining the capacity required is a two-step process. First, the time needed for
each order at each work center is determined. Second, the capacity required for individual
orders to obtain the load is totaled.
Time Needed for Each Order
The time needed for each order is the sum of the setup time and the run time. The run time
is equal to the run time per piece multiplied by the number of pieces in the order.
example Problem
A work center is to process 150 units of gear shaft SG 123 on work order 333. The
setup time is 1.5 hours, and the run time is 0.2 hours per piece. What is the standard
time needed to run the order?
Answer
Total standard time = setup time + run time
= 1.5 + 1150 * 0.22
= 31.5 standard hours
120
Chapter five
example Problem
In the previous problem, how much actual time will be needed to run the order if the
work center has an efficiency of 120% and a utilization of 80%?
Capacity required = 1actual time21efficiency21utilization2
capacity required
Actual time =
1efficiency21utilization2
31.5
=
11.2210.82
= 32.8 hours
Load
The load on a work center is the sum of the required times for all the planned and actual
orders to be run on the work center in a specified period. The steps in calculating load are
as follows:
1. Determine the standard hours of operation time for each planned and released order
for each work center by time period.
2. Add all the standard hours together for each work center in each period. The result is
the total required capacity (load) on that work center for each time period of the plan.
example Problems
A work center has the following open orders and planned orders for week 20. Calculate
the total standard time required (load) on this work center in week 20. Order 222 is
already in progress, and there are 100 remaining to run.
Order
Quantity
Setup
Time (hours)
run Time
(hours/piece)
Total Time
(hours)
Released Orders
222
100
0
0.2
333
150
1.5
0.2
Planned Orders
444
200
3
0.25
555
300
2.5
0.15
Total Time
Answer
Released orders 222
333
Planned orders 444
555
Total Time
Total time = 0 + 1100 * 0.22
= 20.0 standard hours
Total time = 1.5 + 1150 * 0.22 = 31.5 standard hours
Total time = 3 + 1200 * 0.252 = 53.0 standard hours
Total time = 2.5 + 1300 * 0.152 = 47.5 standard hours
= 152.0 standard hours
In week 20, there is a load (requirement) for 152 standard hours.
The load must now be compared to the available capacity. One way of doing this is
with a work center load report.
Work Center Load Report
The work center load report shows future capacity requirements based on released and
planned orders for each time period of the plan.
The load of 152 hours calculated in the previous example is for week 20. Similarly,
loads for other weeks can be calculated and recorded on a load report such as the one
shown in Figure 5.5. Figure 5.6 shows the same data in graphical form. Note that the
Capacity Management
121
CapaCity in serviCes
While few people in manufacturing tend to initially think of
inventory as a luxury (many try to limit it because of the cost)
from a capacity standpoint, it is a luxury. This is because
manufacturing companies often can utilize their capacity
to produce inventory ahead of the actual demand for that
inventory. So, when the demand occurs, they can react very
quickly by taking the inventory from stock. From that perspective, inventory can be thought of as “stored capacity.”
Few service firms have that luxury of using inventory to
store capacity. While many do carry inventory (retail shops, for
example), the capacity they cannot “stock” is the capacity for
the shop clerks to help customers. Since the demand for that
service capacity is often erratic and difficult to predict, how
do services respond while minimizing the time that capacity
might be “wasted” (e.g., when there no customers to serve)?
There are at least three common methods used:
shelves, applying pricing tags, yet is also capable of
serving customer needs. They can, for example, be
restocking shelves when there is no demand for helping a customer, but immediately shift over to helping
a customer when one needs help.
2. Utilize automation. Self-service checkout lines and
ATMs in banks are examples.
3. In cases where the capacity of the service worker
is often limited and very expensive (a physician or
dentist, for example), rather than focus on automation
or flexibility, the service will often control the demand
itself so that the capacity can be highly utilized without pressure. Service appointments and reservations
are the methods commonly used. This approach is also
used in many automobile repair facilities and popular
restaurants for the same reason.
1. Employ multiskilled and flexible workers, e.g.,
a worker who is knowledgeable about restocking
Week
20
21
22
23
24
Total
Released Load
51.5
45
30
30
25
181.5
Planned Load
100.5
120
100
90
100
510.5
Total Load
152
165
130
120
125
692
Rated Capacity
140
140
140
140
140
700
(Over)/Under
Capacity
(12)
(25)
10
20
15
8
Figure 5.5 Work center load report.
Legend
200
Rated Capacity
Planned Load
Released Load
STANDARD HOURS
150
100
50
0
20
21
22
WEEKS
Figure 5.6 Graph of a load profile.
23
24
122
Chapter five
report shows released and planned load, total load, rated capacity, and over/undercapacity. The term overcapacity means that the work center is overloaded and the term undercapacity means the work center is underloaded. This type of display gives information
used to adjust available capacity or to adjust the load by changing the priority plan. In
this example, weeks 20 and 21 are overloaded, the rest are underloaded, and the cumulative load is less than the available. For the planner, this shows there is enough total
capacity over the planning horizon, and available capacity or priority can be juggled to
meet the plan.
sCheDuling orDers
So far the assumption has been that CRP knows when an order should be run on one work
center. Most orders are processed across a number of work centers, and it is necessary
to calculate when orders must be started and completed on each work center so the final
due date can be met. This process is called scheduling. APICS Dictionary, 14th edition
defines a schedule as “a timetable for planned occurrences.”
Back scheduling The usual process is to start with the due date and, using the lead
times, to work back to find the start date for each operation. This process is called
back scheduling or backward scheduling. To back schedule, the following must be
known for each order:
Quantity and due date.
Sequence of operations and work centers needed.
■■ Setup and run times for each operation.
■■ Queue, wait, and move times.
■■ Work center capacity available (rated or demonstrated).
■■
■■
The information needed is obtained from the following:
Planned and open orders: Quantities and due dates.
Routing: Sequence of operations, work centers needed, setup time, and run time.
■■ Work center data: Queue, move, and wait times and work center capacity.
■■
■■
The process is as follows:
1. For each work order, calculate the capacity required (time) at each work center.
2. Starting with the due date, schedule back to get the completion and start dates for
each operation.
example Problem
Suppose there is an order for 150 gear shaft SG 123. The due date is day 135. The
route sheet, shown in Figure 5.3, gives information about the operations to be performed and the setup and run times. The work center file, shown in Figure 5.7, gives
lead time data for each work center. Calculate the start and finish dates for each
operation. Use the following scheduling rules.
Work
Center
Queue Time
(days)
Wait Time
(days)
Move Time
(days)
12
14
17
03
4
3
5
8
1
1
1
1
1
1
1
1
Figure 5.7 Lead time data from work center file.
Capacity Management
123
Operation
Number
Work
Center
Arrival
Date
(a.m.)
Queue
(days)
Operation
(days)
Wait
(days)
Finish
Date
(p.m.)
10
12
95
4
4
1
103
20
14
105
3
5
1
113
30
17
115
5
1
1
121
40
03
123
8
2
1
133
50
Stores
135
Figure 5.8 Work schedule.
a. Operation times are rounded up to the nearest 8 hours and expressed as days on a
one-shift basis. That is, if an operation takes 6.5 standard hours, round it up to 8
hours, which represents one day.
b. Assume an order starts at the beginning of the day and finishes at the end of a
day. For example, if an order starts on day 1 and is finished on day 5, it has taken
5 days to complete. If move time is 1 day, the order will be available to the next
workstation at the start of day 7.
Answer
The calculations for the operation time at each work center are as follows:
Setup time + run time = total time 1standard hours2
Operation 10: Work center 12: 1.5 + 0.20 * 150 = 31.5 standard hours
= 4 days
Operation 20: Work center 14: 0.50 + 0.25 * 150 = 38.0 standard hours
= 5 days
Operation 30: Work center 17: 0.30 + 0.05 * 150 = 7.8 standard hours
= 1 day
Operation 40: Work center 03: 0.45 + 0.10 * 150 = 15.45 standard hours
= 2 days
The next step is to schedule back from the due date (day 135) to get the completion
and start dates for each operation. To do so, not only must the operation times just calculated be known, but also the queue, wait, and move times. These are part of the work
center data, such as those shown in Figure 5.7.
The back scheduling process starts with the last operation. The goods are to be in the
stores at the beginning of day 135. It takes 1 day to move them, so the order must be completed on operation 40 at the end of day 133, leaving day 134 to move the product. Subtracting
the wait, queue, and operation times (11 days), the order must be started at the beginning of
day 123. With a move time of 1 day, it must be completed on operation 30 at the end of day
121. Using this process, the start and completion dates can be calculated for all operations.
Figure 5.8 shows the resulting schedule and Figure 5.9 shows the same thing graphically.
An alternative approach is to use forward scheduling, which begins with a start date
at the first operation, and moves forward through the operations calculating the start and
completion dates for each operation to determine the completion date.
Making the plan
So far the data needed for a capacity requirements plan, where the data comes from, and
the scheduling and loading of shop orders through the various work centers has been
determined. The next step is to compare the load with available capacity to see if there are
imbalances and if so, to find possible solutions.
124
Chapter five
Work
Center
12
Operation 10
Operation 20
14
Operation 30
17
Operation 40
03
Stores
90
100
110
120
Release
Date
130
135
Due
Date
Figure 5.9 Graphical work schedule.
There are two ways of balancing capacity available and load: alter the load, or change
the capacity available. Altering the load means shifting orders ahead or back so the load is
leveled. If orders are processed on other work stations, the schedule and load on the other
work stations have to be changed as well. It may also mean that other components should
be rescheduled and the master production schedule changed.
Consider the bill of material shown in Figure 5.10. If component B is to be rescheduled to a later date, then the priority for component C is changed, as is the master production schedule for A. For these reasons, changing the load is usually not the preferred
course of action. In the short timeframe, capacity can usually be adjusted easier than load.
Some ways that this may be done are as follows:
Schedule overtime or undertime. This will provide a temporary solution for cases
where the load/capacity imbalance is not too large or long term.
■■ Adjust the level of the workforce by hiring or laying off workers. The ability to do
so will depend on the availability of the skills required and the training needed. The
higher the skill level and the longer the training needed, the more difficult it becomes
to change the level of the workforce quickly.
■■ Shift workforce from underloaded to overloaded work centers. This may require a
flexible cross-trained workforce, or adaptable equipment.
■■ Use alternate routings to shift some load to another work center. Often the other work
center is not as efficient as the original. Nevertheless, the important thing is to meet
the schedule, and this is a valid way of doing so.
■■ Subcontract work when more capacity is needed or bring in previously subcontracted
work to increase load. It may be more costly to subcontract rather than make the item
in-house, but again, it is important to maintain the schedule.
■■
The result of capacity requirements planning should be a detailed workable plan that
meets the priority objectives and provides the capacity to do so. Ideally, it will satisfy the
material requirements plan and allow for adequate utilization of the workforce, machinery, and equipment.
A
B
Figure 5.10 Simple bill of material.
C
Capacity Management
125
suMMary
Capacity management occurs at all levels of the planning process. It is directly related to
the priority plan, and the level of detail and time spans will be similar to the related priority plan.
Capacity planning is concerned with translating the priority plan into the hours of capacity required in manufacturing to make the items in the priority plan and with methods
of making that capacity available. Capacity available depends upon the number of workers
and machines, their utilization, and efficiency.
Capacity requirements planning occurs at the material requirements planning level. It
takes open shop orders and planned orders from the MRP and converts them to a load on
each work center. It considers lead times and actual order quantities. It is the most detailed
of the capacity planning techniques.
Material requirements planning and capacity requirements planning should form part
of a closed-loop system that not only includes planning and control functions but also
provides feedback so planning can always be current. Figure 5.11 illustrates the concept.
MPS
No
FEEDBACK
CAPACITY OKAY?
(ROUGH CUT)
Yes
No
FEEDBACK
MRP
CAPACITY OKAY?
(CRP)
Yes
PRODUCTION
ACTIVITY
CONTROL
PURCHASING
PERFORMANCE MEASURES
Figure 5.11 MRP and CRP closed-loop system.
key terMs
Available time 117
Back scheduling 122
Backward scheduling 122
Calculated capacity 118
Capacity 112
Capacity available 112
Capacity control 113
Capacity cushion 119
Capacity management 112
Capacity planning 113
126
Chapter five
Capacity required 112
Capacity requirements planning (CRP) 114
Demonstrated (measured) capacity 118
Efficiency 117
Forward scheduling 123
Lead time 115
Load 112
Load report 120
Move time 115
Open order 115
Planned orders 115
Productivity 118
Queue time 115
Rated capacity 118
Resource planning 113
Rough-cut capacity planning (RCCP) 113
Routing 115
Safety capacity 119
Scheduling 122
Shop calendar 116
Standard hours 117
Standard time 117
Theoretical capacity 118
Utilization 117
Wait time 115
Work center 115
questions
1. What are the responsibilities of capacity management?
2. What is capacity planning?
3. Describe the three steps of capacity planning.
4. Relate the three levels of priority planning to capacity planning. Describe each level in terms of
the detail and the time horizons used.
5. What is capacity requirements planning? At what level of the priority planning process does it
occur?
6. What are the inputs to the CRP process? Where is this information obtained?
7. Describe each of the following and the information they contain.
a. Open order.
b. Routing.
c. Work center.
8. What is a shop calendar? Why is it needed?
9. Where would you find the following information?
a. A scheduled receipt.
b. A planned receipt.
c. Efficiency and utilization.
d. Sequence of operations on a part.
10. Define capacity available. What are the four factors that affect it?
11. Why is standard time usually used to measure capacity?
12. What are theoretical capacity, rated capacity, utilization, and efficiency? How are they related?
13. What is measured or demonstrated capacity? How is it different from rated capacity?
14. What is load?
15. What is a work center load report? What information does it contain?
16. What is a schedule?
17. Describe the process of backward scheduling.
18. What are the two ways of balancing capacity available and load? Which is preferred? Why?
19. What are some of the ways capacity available can be altered in the short run?
20. Why is feedback necessary in a control system?
21. What might be some of the problems in scheduling rated capacity too closely to the load?
22. How is safety capacity used?
Capacity Management
127
probleMs
5.1. A work center consists of 3 machines each working a 16-hour day for 5 days a week.
What is the weekly available time?
Answer.
240 hours per week
5.2. The work center in problem 5.1 is utilized 75% of the time. What are the hours per
week actually worked?
5.3. If the efficiency of the work center in problem 5.1 is 120%, what is the rated capacity of the work center?
Answer.
216 standard hours per week
5.4. A work center consisting of 7 machines is operated 16 hours a day for a 5-day week.
Utilization is 80%, and efficiency is 110%. What is the rated weekly capacity in
standard hours?
Answer.
492.8 standard hours per week
5.5. A work center consists of 4 machines working 8 hours a day for a 5-day week. If the
utilization is 80% and the efficiency is 90%, what is the rated capacity of the work
center?
5.6. Over a period of 4 weeks, a work center produced 50, 45, 42, and 52 standard hours
of work. What is the demonstrated capacity of the work center?
Answer.
47.25 standard hours of work per week
5.7. In an 11-week period, a work center produces 1050 standard hours of work. What is
the measured capacity of the work center?
5.8. In 1 week, a work center produces 75 standard hours of work. The hours scheduled
are 80, and 72 hours are actually worked. Calculate the utilization and efficiency of
the work center.
Answer.
Utilization is 90%; efficiency is 104%
5.9. A work center consisting of 3 machines operates 40 hours a week. In a 4-week
period, it actually worked 355 hours and produced 475 standard hours of work.
Calculate the utilization and efficiency of the work center. What is the demonstrated
weekly capacity of the work center?
5.10. A firm wishes to determine the efficiency and utilization of a work center composed
of 5 machines each working 16 hours per day for 5 days a week. A study undertaken
by the materials management department found that over the past 50 weeks the work
center was actually working for 16,000 hours, and work performed was 15,200 standard hours. Calculate the utilization, efficiency, and demonstrated weekly capacity.
5.11. How many standard hours are needed to run an order of 200 pieces if the setup time
is 1.3 hours and the run time 0.3 hours per piece? How many actual hours are needed
at the work center if the efficiency is 130% and the utilization is 70%?
Answer.
61.3 standard hours; 67.4 actual hours
5.12. How many standard hours are needed to run an order of 500 pieces if the setup time
is 2.0 hours and the run time 0.3 hours per piece? How many actual hours are needed
at the work center if the efficiency is 110% and the utilization is 85%?
5.13. A work center has the following open and planned orders for week 4. Calculate the
total standard time required (load).
128
Chapter five
Released Orders
Planned Orders
Order
Quantity
Setup Time
(hours)
Run Time
(hours/piece)
120
300
1.00
0.10
340
200
2.50
0.30
560
300
3.00
0.25
780
500
2.00
0.15
Total Time
(hours)
Total Time (standard hours)
Answer.
Total time = 248.5 standard hours
5.14. A work center has the following open and planned orders for week 4. Calculate the
total standard time required (load).
Released Orders
Planned Orders
Order
Quantity
Setup Time
(hours)
Run Time
(hours/piece)
125
200
0.50
0.10
345
70
0.75
0.05
565
80
1.00
0.25
785
35
1.50
0.15
Total Time
(hours)
Total Time (standard hours)
5.15. Using the following route information, open order information, and MRP planned
orders, calculate the load on the work center.
Routing:
Part 123:
Part 456:
Setup time
= 2 standard hours
Run time per piece = 3 standard hours per piece
Setup time
= 3 standard hours
Run time per piece = 1 standard hour per piece
Open Orders for parts
Planned Orders for parts
Week
1
2
3
1
2
3
123
12
8
5
0
5
10
456
15
5
5
0
10
15
Capacity Management
129
Load report
Week
1
Released Load
2
3
123
456
Planned Load
123
456
Total Load
5.16. Complete the following load report and suggest possible courses of action.
Week
18
19
20
21
Total
Released Load
160
155
100
70
485
Planned Load
0
0
70
80
150
150
150
150
150
600
Total Load
Rated Capacity
(Over)/Under
Capacity
5.17. Back schedule the following shop order. All times are given in days. Move time
between operations is 1 day, and wait time is 1 day. Due date is day 150. Assume
orders start at the beginning of a day and finish at the end of a day.
Operation
Number
Work
Center
Operation
Time (days)
Queue Time
(days)
10
111
2
4
20
130
4
5
30
155
1
2
Stores
Answer.
Arrival
Date
(a.m.)
150
The order must arrive at work center 111 on day 126.
Finish
Date
(p.m.)
130
Chapter five
5.18. Back schedule the following shop order. All times are given in days. Move time
between operations is 1 day, and wait time is 1 day. Due date is day 200. Assume
orders start at the beginning of a day and finish at the end of a day.
Operation
Number
Work
Center
Operation
Time (days)
Queue Time
(days)
10
110
3
2
20
120
5
3
30
130
3
2
Stores
Arrival
Date
(a.m.)
Finish
Date
(p.m.)
200
Case stuDy 5.1
Wescott Products
Whenever Jason Roberts thought about going to work on Friday morning, he started to
get a little knot in his stomach. Jason had recently accepted the job as operations manager
for a small manufacturing company that specialized in a line of assemble-to-order products. When he accepted the job he was a recent graduate of a business program where he
specialized in operations. He had done fairly well in his classes and had emerged as a confident, self-assured person who was sure he could handle such a job in a small company.
The company, Wescott Products, had recently experienced rapid growth from the
original start in a two-car garage just five years earlier. In fact, Jason was the first person
ever named as operations manager. Prior to that, the only production manager reporting to
the owner, Judy Wescott, was Frank Adams, the production supervisor. While Frank was
an experienced supervisor, he had been promoted to supervisor directly from his old job as
a machine operator and had no formal training in planning and control. He soon found that
planning was too complex and difficult for him to handle, especially since he also had full
responsibility for all the Wescott workers and equipment. Randy Stockard, the sales and
marketing manager, had requested and finally applauded Judy Wescott’s decision to hire
Jason, since he felt production was having a much more difficult time in promising and
delivering customer orders. Randy was starting to spend more and more time on the phone
with angry customers when they didn’t get their orders at the time they expected them.
The time away from developing new sales and the danger of losing established customers
started to make him highly concerned about sustaining sales growth, to say nothing about
his potential bonus check tied to new sales!
Once Jason was placed in the position, however, the “honeymoon” was short, and
soon Jason started doubting how much he really did know. The company was still having trouble with promising customer orders and having the capacity to meet those orders.
At first he thought it was the forecasting method he used, but a recent analysis told him
the total actual orders were generally within 10% of what the forecast projected. In addition, production never seemed to have any significant shortages in either subassemblies
or components. In fact, many felt they had far too much material, and in the last couple
of staff meetings Jake Marris, the company controller, was grumbling that he thought the
inventory turn ratio of just less than 3.5 was unreasonable and costing the company a lot
of money. It must be something else, and he had to discover it quickly.
The first idea he thought about was to request the assembly areas to work overtime,
but he soon found out that was a sensitive topic that could only be used as a last resort.
The workers in that area were highly skilled and would be difficult, if not impossible, to
Capacity Management
131
replace in any reasonable time. Adding more employees would also be difficult for the
same reason. A year earlier they were being worked a lot of overtime but had finally had
enough. Even though Wescott had no union, the workers got together and demanded better overtime control or they would all quit to move to other jobs that were plentiful for
skilled workers in this area. The agreement was that they were to be asked for no more
than four hours of overtime per worker per week unless it was truly an emergency situation. They were well paid and all had families, and the time with their families was worth
more to them than additional overtime pay. At least the high skill level had one advantage:
Each of the workers in the assembly area could skillfully assemble any of the models, and
the equipment was flexible enough to handle all the models.
Friday mornings were when Jason made his master schedule for the next week and no
matter how hard he tried he never seemed to be able to get it right. Since the standard lead
time for all assemblies was quoted as one week, the company had felt no need to schedule
farther into the future when very few orders existed there. He was sure that he had to start
the process by loading the jobs that were missed in the current week into the Monday and
Tuesday time blocks and then hope that production could catch up with those in addition to the new jobs that were already promised. The promises came when Randy would
inform him of a customer request and ask for a promise date, which was often “as soon
as possible.” Jason would look at the order to see if the material to make it was in stock
and if the equipment to make it was running. He would then typically promise to have it
available when requested. Now that a lot of promises were not being met, however, Randy
was starting to demand that Jason get control of the operation. Jason tried to respond by
scheduling a lot of each model to be run every week, but he often found he had to break
into the run of a lot to respond to expediting from sales. He knew this made matters worse
by using extra time to set up the equipment, but what else could he do? Even Judy Wescott
was asking him what she needed to do to help him improve the performance. His normal
high level of self-confidence was being shaken.
Jason started pouring over his old operations book looking for something he could
use. He finally realized that what he needed was a more effective system to develop master schedules from which he could promise orders, order components, and plan capacity.
Unfortunately, he also recalled that when that material was covered in his class he had
taken off early for spring break. Even though he knew enough to recognize the nature
of the problem, he didn’t know enough to set up such a schedule. Humbly, he called his
former instructor to ask for advice. Once she was briefed on the problem, she told him to
gather some information that he could use to develop a sample master schedule and roughcut capacity plan. Once he had the information, she would help show him how to use it.
The following describes what she asked him to collect:
1. Pick a work center or piece of equipment that has caused some capacity problem in
the recent past. List all the product models that use that work center.
2. For each of the models, list the amount of run time they use the work center per item.
Also list the setup time, if any. These times can be gathered from standards or, if the
standard data is suspect or does not exist, use the actual average time from recent
production.
3. For each of the models, list the usual lot size. This should be the same lot size used for
the master schedule.
4. For each of the models, list the current inventory, the current forecast, and the current
firm customer order quantities.
5. List the current capacity (hours) available for the equipment.
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Chapter five
The following tables summarize the data Jason collected:
Model
A
B
C
D
E
Work Center 12
Run Time
Lot Size
(per item,
Setup Time (minimum
in minutes)
(per lot)
qty.)
On-hand*
3.7
90 minutes
150
10
5.1
40 minutes
100
0
4.3
60 minutes
120
0
8.4
200 minutes
350
22
11.2
120 minutes
400
153
*Most of the on-hand was really forced when the lot size exceeded orders for the week
for that model. They would then assemble the rest of the lot as “plain vanilla,” such that
they could easily add any subassembly options once the actual customer orders came in.
Two workers are currently assigned to the work center, and only to the first shift.
Even though assembly workers are very flexible, Jason cannot take workers from another
assembly area, as those work centers are also behind and therefore appear to be equally
overloaded.
The following is the forecast and customer orders for each of the five models assembled in work center 12:
Model Weeks
A
Forecast
Customer Orders
B
Forecast
Customer Orders
C
Forecast
Customer Orders
D
Forecast
Customer Orders
E
Forecast
Customer Orders
1
45
53
35
66
50
52
180
277
200
223
2
45
41
35
40
50
43
180
190
200
174
3
45
22
35
31
50
33
180
178
200
185
4
5
6
7
8
9
10
45
45 45 45 45 45 45
15
4
7
2
0
0
0
35
35 35 35 35 35 35
30
17
6
2
0
0
0
50
50 50 50 50 50 50
21
14
4
7
1
0
0
180 180 180 180 180 180 180
132
94 51 12
7
9
2
200 200 200 200 200 200 200
109
74 36 12
2
0
0
Once Jason had gathered all the data, he immediately called his instructor, only to find
out that by an ironic twist of fate she would be gone for more than a week on spring break.
assignment
This leaves you to help Jason. Specifically, you need to do the following:
1. Discuss the nature and probable sources of the problem.
2. Examine the rough-cut capacity situation using the data Jason gathered. Discuss the
results and how they are linked to the problems identified in question 1.
3. Use the information and your knowledge of the situation to develop a complete plan
for Jason to use in the future. Part of this plan should be to build and demonstrate the
approach to master scheduling for the data given in the case.
Chapter
six
Production Activity
control
introduction
After all the planning and scheduling, it is time for the plans to be put into action. Production
activity control (PAC) is responsible for executing the master production schedule (MPS)
and the material requirements plan (MRP). At the same time, it must make good use of labor
and machines, minimize work-in-process inventory, and maintain customer service.
The material requirements plan authorizes PAC:
To release work orders to the shop for manufacturing.
To manage work orders and make sure they are completed on time.
■■ To be responsible for the immediate detailed planning of the flow of orders through
manufacturing, carrying out the plan, and controlling the work as it progresses to
completion.
■■ To manage day-to-day activity and provide the necessary support.
■■
■■
Figure 6.1 shows the relationship between the planning system and PAC.
The activities of the PAC system can be classified into planning, implementation, and
control functions.
PRODUCTION
PLANNING
PLANNING
MASTER
PRODUCTION
SCHEDULE
MATERIAL
REQUIREMENTS
PLAN
IMPLEMENTATION
AND
CONTROL
PURCHASING
PRODUCTION
ACTIVITY
CONTROL
INPUT/
OUTPUT
CONTROL
OPERATION
SEQUENCING
Figure 6.1 Priority planning and production activity control.
133
134
Chapter six
Planning
The flow of work through each of the work centers must be planned to meet delivery
dates, which means production activity control must do the following:
Ensure that the required materials, tooling, personnel, and information are available to
manufacture the components when needed.
■■ Schedule start and completion dates for each shop order at each work center so the
scheduled completion date of the order can be met. This will involve the planner in
developing a load profile for the work centers.
■■
Implementation
Once the plans are made, production activity control must put them into action by advising the shop floor what must be done. Instructions can be given by issuing a shop order
with the relevant information, or by simply producing a schedule that shows product
information, quantities, and dates. Production activity control will:
■■
■■
Gather the information needed by the shop floor to make the product.
Release orders to the shop floor as authorized by the material requirements plan. This
is called dispatching.
Control
Once plans are made and shop orders released, the process must be monitored to learn
what is actually happening. The results are compared to the plan to decide whether corrective action is necessary. Production activity control will do the following:
Rank the shop orders in desired priority sequence by work center and establish a dispatch list based on this information.
■■ Track the actual performance of work orders and compare it to planned schedules.
Where necessary, PAC must take corrective action by replanning, rescheduling, or
adjusting capacity to meet final delivery requirements.
■■ Monitor and control work-in-process, lead times, and work center queues.
■■ Report work center efficiency, operation times, order quantities, and scrap.
■■
The functions of planning, implementing, and controlling are shown in Figure 6.2.
PRODUCTION ACTIVITY CONTROL
PLAN
Schedule
Replan
IMPLEMENT
Work
Authorization
CONTROL
Compare
Decide
Dispatch
MANUFACTURING OPERATIONS
Figure 6.2 Schematic of a production control system.
Feedback
Production Activity Control
135
Manufacturing Systems
The particular type of production control system used varies from company to company,
but all should perform the functions just mentioned. However, the relative importance of
these functions will depend on the type of manufacturing process. Manufacturing processes can be conveniently organized into three categories:
1. Flow manufacturing.
2. Intermittent manufacturing.
3. Project manufacturing.
Flow manufacturing Flow manufacturing is concerned with the production of high
volume standard products. If the units are discrete (e.g., cars and appliances), the process
is usually called repetitive manufacturing, and if the goods are made in a continuous
flow (e.g., gasoline), the process is called continuous manufacturing. There are four
major characteristics of flow manufacturing:
1. Routings are fixed, and work centers are arranged according to the routing. The time
taken to perform work at one work center is almost the same as at any other work
center in the line, enabling a constant flow.
2. Work centers are dedicated to producing a limited range of similar products.
Machinery and tooling are especially designed to make the specific products.
3. Material flows from one workstation to another using some form of mechanical transfer. There is little buildup in work-in-process inventory, and throughput times are low.
4. Capacity is fixed by the line.
Production activity control concentrates on planning the flow of work and making
sure that the right material is fed to the line as stated in the planned schedule. Since work
flows from one workstation to another automatically, implementation and control are relatively simple.
Intermittent manufacturing Intermittent manufacturing is characterized by many
variations in product design, process requirements, and order quantities. This kind of manufacturing is characterized by the following:
1. Flow of work through the shop is varied and depends on the design of a particular
product. As orders are processed, they may take more time at one workstation than at
another. Thus, the work flow is not balanced.
2. Machinery and workers must be flexible enough to do the variety of work involved in
intermittent manufacturing. Machinery and work centers are usually grouped according to the function they perform, for example, all lathes in one department.
3. Throughput times are generally long. Scheduling work to arrive just when needed
is difficult, the time taken by an order at each work center varies, and work queues
before work centers, causing long delays in processing. Work-in-process inventory is
often large.
4. The capacity required depends on the particular mix of products being built and is
sometimes difficult to predict.
Production activity control in intermittent manufacturing is complex. Because of the
number of products made, the variety of routings, and scheduling problems, PAC is a
major activity in this type of manufacturing. Planning and control are typically exercised
using shop orders or detailed schedules for each batch being produced. Most of the discussion of PAC in this text assumes this kind of environment.
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Chapter six
Project manufacturing Project manufacturing usually involves the creation of one
unit or a small number of units. Complex shipbuilding is an example. Because the design
of a product is often carried out or modified as the project develops, there is close coordination between manufacturing, marketing, purchasing, and engineering.
Project manufacturing or management uses many of the same techniques as production activity control, but also has some unique characteristics. Activities typically included
in project management are:
Initiating the project, which includes identifying the project requirements
■■ Planning the project, including the scope, schedule and tasks, budget, resources, and risks
■■ Executing the project by carrying out the tasks
■■ Monitoring and controlling the project tasks and resources, and communicating the
status of the project to stakeholders
■■ Closing the project, which includes documenting the results, as well as any variances
in time and costs
■■
dAtA requirements
To plan the processing of materials through manufacturing, PAC must have the following
information:
What and how much to produce.
When parts are needed so the completion date can be met.
■■ What operations are required to make the product and how long the operations will take.
■■ What the available capacities of the various work centers are.
■■
■■
Production activity control must have a data or information system from which to
work. Usually the data needed to answer these questions is organized into databases. The
information contained in the databases are of two types: planning and control.
Planning Information
Four types of planning information are needed: item master, product structure, routing,
and work center master.
Item master There is one record in the item master database for each part number,
containing all of the pertinent data related to the part. For PAC, this includes the following:
Part number. A unique number assigned to a component.
Part description.
■■ Manufacturing lead time. The normal time needed to make this part.
■■ Lot-size quantity. The quantity normally ordered at one time.
■■
■■
Product structure (bill of material) The product structure (bill of material) contains a list of the single-level components and quantities needed to assemble a parent. It
forms a basis for a pick list to be used by storeroom personnel to collect the parts required
to make the assembly. There are a variety of ways of displaying a bill of material, including a single level bill of material, an indented bill of material, transient bill of material,
matrix bill of material, and costed bill of material. In some industries, in particular the
process industry, the bill of material is called a formula, recipe, or ingredients list.
Routing The routing contains a record for each part manufactured. The routing consists
of a series of operations required to make the item. For each product, a step-by-step set of
instructions is provided describing how the product is made. It gives details of the following:
The operations required to make the product and the sequence in which those operations are performed.
■■ A brief description of each operation.
■■
Production Activity Control
137
Equipment, tools, and accessories needed for each operation.
Setup times, the standard time required for setting up the equipment for each
operation.
■■ Run times, the standard time required to process one unit through each operation.
■■ Lead times for each operation.
■■
■■
Work center master The work center master collects all of the relevant data on a
work center. For each work center, it gives details on the following:
Work center number.
Capacity.
■■ Number of shifts worked per week.
■■ Number of machine hours per shift.
■■ Number of labor hours per shift.
■■ Efficiency.
■■ Utilization.
■■ Queue time, the average time that a job waits at the work center before work is begun.
■■ Alternate work centers, work centers that may be used as alternatives.
■■
■■
Control Information
Control in intermittent manufacturing is exercised through shop orders and the data contained on these orders.
Shop order master Each active manufacturing order has a record in the shop order
master. The purpose is to provide summarized data on each shop order, such as the following information:
Shop order number, a unique number identifying the shop order.
Order quantity.
■■ Quantity completed.
■■ Quantity scrapped.
■■ Quantity of material issued to the order.
■■ Due date, the date the order is expected to be finished.
■■ Priority, a value used to rank the order in relation to others.
■■ Balance due, the quantity not yet completed.
■■ Cost information.
■■
■■
The detailed records for each shop order contain records for each operation needed to
make the item. Each record contains the following information:
Operation number.
Setup hours, planned and actual.
■■ Run hours, planned and actual.
■■ Quantity reported complete at that operation.
■■ Quantity reported scrapped at that operation.
■■ Due date or lead time remaining.
■■
■■
order PrePArAtion
Once authorization to process an order has been received, production activity control is
responsible for planning and preparing its release to the shop floor. The order should be
reviewed to be sure that the necessary tooling, material, and capacity are available. If they
are not, the order cannot be completed and should not be released.
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Chapter six
If MRP software is used, it will have checked the availability of material and preallocated it to a shop order so no further checking is necessary. If MRP software is not
used, production activity control must manually check material availability of all the components necessary to produce the goods.
If a capacity requirements planning system has been used, necessary capacity should
be available. However, at this stage, there may be some differences between planned
capacity and what is actually available, due to product that is behind schedule, daily
changes in workforce, and so forth. When capacity requirements planning is not used, it is
necessary to determine if capacity is available before releasing orders.
Checking capacity availability is a two-step process. First, the order must be scheduled to see when the capacity is needed, and second, the load on work centers must be
checked in that period.
scheduling
The objective of scheduling is to meet delivery dates and to make the best use of manufacturing resources. It involves establishing start and finish dates for each operation required
to complete an item. To develop a reliable schedule, the planner must have information
on the routing, required and available capacity, competing jobs, and manufacturing lead
times at each work center involved.
Manufacturing Lead Time
Manufacturing lead time is the time normally required to produce an item in a typical lot
quantity. Typically, it consists of five elements:
1. Queue time, amount of time the job is waiting at a work center before operation
begins.
2. Setup time, time required to prepare the work center for operation.
3. Run time, time needed to run the order through the operation.
4. Wait time, amount of time the job is at the work center before being moved to the
next work center.
5. Move time, transit time between work centers.
The total manufacturing lead time will be the sum of order preparation and release
plus the manufacturing lead time for each operation. Figure 6.3 shows the elements making
up manufacturing lead time. Setup time and run time are straightforward, and determining
them is the responsibility of the industrial engineering department. Queue, wait, and move
times are under the control of manufacturing and PAC.
The largest of the five elements is queue time. Typically, in an intermittent manufacturing operation, it may account for 85–95% of the total lead time. Production activity
control is responsible for managing the queue by regulating the flow of work into and out
of work centers. If the number of orders waiting to be worked on (load) is reduced, so
ORDER
RELEASE
QUEUE
SETUP
RUN
WAIT
QUEUE
SETUP
RUN
WAIT
Figure 6.3 Manufacturing lead time.
MOVE
MOVE
QUEUE
SETUP
RUN
WAIT
QUEUE
SETUP
RUN
WAIT
MOVE
MOVE
Production Activity Control
139
is the queue time, the lead time, and work-in-process. Increasing capacity also reduces
queue. Production activity control must manage both the input of orders to the production
process and the available capacity to control queue and work-in-process.
A term that is closely related to manufacturing lead time is cycle time. APICS
Dictionary, 14th edition defines cycle time as “the length of time from when material
enters a production facility until it exits.” A synonym of cycle time is throughput time.
example Problem
An order for 100 of a product is processed on work centers A and B. The setup time
on A is 30 minutes, and run time is 10 minutes per piece. The setup time on B is 50
minutes, and the run time is 5 minutes per piece. Wait time between the two operations is 4 hours. The move time between A and B is 10 minutes. Wait time after operation B is 4 hours, and the move time into stores is 15 minutes. There is no queue at
either workstation. Calculate the total manufacturing lead time for the order.
Answer
Work center A operation time = 30 + 1100 * 102
Wait time
Move time from A to B
Work center B operation time = 50 + 1100 * 52
Wait time
Move time from B to stores
Total manufacturing lead time
= 1030 minutes
= 240 minutes
=
10 minutes
= 550 minutes
= 240 minutes
=
15 minutes
= 2085 minutes
=
34 hours, 45 minutes
Scheduling Techniques
There are many techniques used to schedule shop orders through a plant, but all of them
require an understanding of forward and backward scheduling as well as finite and infinite
loading.
Forward scheduling assumes that material procurement and operation scheduling for a component start when the order is received, whatever the due date, and that
operations are scheduled forward from this date. The first line in Figure 6.4 illustrates
this method. The result is completion before the due date, which usually results in
a buildup of inventory. This method is used to decide the earliest delivery date for a
product.
Forward scheduling is used to calculate how long it will take to complete a task. The
technique is used for purposes such as developing promise dates for customers or figuring
out whether an order behind schedule can be caught up.
Order Received
1
2
Due Date
3
4
5
6
7
8
9
Forward Scheduling
Material
Ordered
1st
operation
2nd
operation
3rd
operation
Backward Scheduling
Material
Ordered
1st
operation
Figure 6.4 Forward and backward scheduling: infinite loading.
2nd
operation
3rd
operation
140
Chapter six
Capacity Overload
CAPACITY
Capacity Underload
Figure 6.5 Infinite load profile.
Backward scheduling is illustrated by the second line in Figure 6.4. The last operation on the routing is scheduled first and is scheduled for completion at the due date.
Previous operations are scheduled back from the last operation. This schedules items to be
available as needed and uses the same logic as the MRP system. Work-in-process inventory
is reduced, but because there is little slack time in the system, customer service may suffer.
Backward scheduling is used to determine when an order must be started. Backward
scheduling is common in a make-to-stock environment because it reduces inventory.
Infinite loading is also illustrated in Figure 6.4. The assumption is made that the
workstations in which operations 1, 2, and 3 are processed have capacity available when
required. It does not consider the existence of other shop orders competing for capacity at
these work centers. It assumes infinite capacity will be available. Figure 6.5 shows a load
profile for infinite capacity. Notice the over- and underload.
Finite loading assumes there is a defined limit to available capacity at any workstation. If there is not enough capacity available at a workstation because of other shop orders,
the order has to be scheduled in a different time period. Figure 6.6 illustrates the condition.
In the forward scheduling example shown in Figure 6.6, the first and second operations cannot be performed at their respective workstations when they should be because
the required capacity is not available at the time required. These operations must be
rescheduled to a later time period. Similarly, in the example of back scheduling, the second and first operations cannot be performed when they should be and must be rescheduled to an earlier time period. Figure 6.7 shows a load profile for finite loading. Notice the
load is smoothed so there is no overload condition.
Order Received
1
2
Due Date
3
4
5
6
7
8
9
Forward Scheduling
Material
Ordered
1st
operation
2nd
3rd
operation operation
Backward Scheduling
Material
Ordered
1st
operation
2nd
operation
Figure 6.6 Forward and backward scheduling. Finite loading.
3rd
operation
Production Activity Control
141
CAPACITY
Smoothed Load
Figure 6.7 Finite load profile.
Chapter 5 gives an example of backward scheduling as it relates to capacity requirements planning. The same process is used in PAC.
example Problem
A company has an order for 50 brand X to be delivered on day 100. Draw a backward
schedule based on the following:
a. Only one machine is assigned to each operation
b. The factory works one 8-hour shift 5 days a week
c. The parts move in one lot of 50.
X
A
B
Part
Operation
Time (days)
A
10
5
A
20
3
B
10
10
X
Assembly
5
Answer
Part A
OP 10
OP 20
X
Assembly
Part B
OP 10
85
90
95
100
WORKING DAYS
Operation Overlapping
In operation overlapping, the next operation is allowed to begin before the entire lot
is completed on the previous operation. This reduces the total manufacturing lead times
because the second operation starts before the first operation finishes all the parts in the
order. Figure 6.8 shows schematically how it works and the potential reduction in lead time.
A concept used in operation overlapping is the difference between a process batch
and a transfer batch. A process batch is the total lot size that has been released to production. A transfer batch is that quantity that moves from work center to work center. A
process batch may consist of one or more transfer batches.
To perform operation overlapping, an order is divided into at least two lots. When the
first lot is completed on operation A, it is transferred to operation B. As seen in Figure 6.8,
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Chapter six
Operation A
SU
LOT 1
LOT 2
T
T
SU
LOT 1
Transit Time
LOT 2
Operation B
Figure 6.8 Operation overlapping.
it is assumed that operation B cannot be set up until the first lot is received, but this is not
always the case. While operation A continues with the second lot, operation B starts on the
first lot. When operation A finishes the second lot, it is transferred to operation B. If the
lots are sized properly, there will be no idle time at operation B. The manufacturing lead
time is reduced by the overlap time and the elimination of queue time.
Operation overlapping is a method of expediting an order, but there are some costs
involved. First, move costs are increased, especially if the overlapped operations are not
close together. Second, it may increase the queue and lead times for other orders. Third, it
does not increase capacity but potentially reduces it if the second operation is idle waiting
for parts from the first operation.
The problem is deciding the size of the sublot. If the run time per piece on operation B is
shorter than that on A, the first batch must be large enough to avoid idle time on operation B.
example Problem
Refer to the data given in the example problem in the section on manufacturing lead
time. It is decided to overlap operations A and B by splitting the lot of 100 into two
lots of 70 and 30. Wait time between A and B and between B and stores is eliminated.
The move times remain the same. Setup on operation B cannot start until the first
batch arrives. Calculate the manufacturing lead time. How much time has been saved?
Answer
Operation time for A for lot of 70 = 30 + 170 * 102 = 730 minutes
10 minutes
Move time between A and B
=
Operation time for B for lot of 100 = 50 + 1100 * 52 = 550 minutes
15 minutes
Move time from B to stores
=
Total manufacturing lead time
= 1305 minutes
=
21 hours, 45 minutes
13 Hours
Time saved
=
Operation Splitting
Operation splitting is a second method of reducing manufacturing lead time. The order is
split into two or more lots or transfer batches and run on two or more machines simultaneously. If the lot is split in two, the run-time component of lead time is effectively cut in
half, although an additional setup is incurred. Figure 6.9 shows a schematic of operation
splitting.
Single Machine
SU
RUN
2 Machine Operation Splitting
SU
RUN
Reduction
in Lead
Time
SU
RUN
Figure 6.9 Operation splitting.
Production Activity Control
143
Operation splitting is practical when:
Setup time is low compared to run time.
A suitable work center is idle.
■■ It is possible for an operator to run more than one machine at a time.
■■ Duplicate tooling or equipment is available.
■■
■■
The last condition often exists when a machine cycles through its operation automatically, leaving the operator time to set up another machine. The time needed to unload and
load must be shorter than the run time per piece. For example, if the unload/load time was
two minutes and the run time was three minutes, the operator would have time to unload
and load the first machine while the second was running.
example Problem
A component made on a particular work center has a setup time of 100 minutes and
a run time of 3 minutes per piece. An order for 500 is to be processed on 2 machines
simultaneously. The machines can be set up at the same time. Calculate the elapsed
operation time.
Answer
Elapsed operation time = 100 + 3 * 250 = 850 minutes
= 14 hours and 10 minutes
loAd leveling
Load profiles were discussed in Chapter 5 in the section on capacity requirements planning. The load profile for a work center is constructed by calculating the standard hours
of operation for each order in each time period and adding them together by time period.
Figure 6.10 shows an example of a load report.
This report tells PAC what the load is on the work center. There is a capacity shortage
in week 20 of 30 hours. This means there would be no point in releasing all of the planned
orders that week. Perhaps some could be released in week 18 or 19, and perhaps some
overtime could be worked to help reduce the capacity crunch.
Work Center: 10
Description: Lathes
Number of Machines: 3
Rated Capacity:
110 standard hours/week
Available Time: 120 hours/week
Efficiency: 115%
Utilization 80%
Week
18
19
20
21
22
23
Total
Released Load
Planned Load
105
100
80
60
30
80
0
130
0
80
315
350
Total Load
105
100
140
110
130
80
665
Rated Capacity
110
110
110
110
110
110
660
(Over)/Under
Capacity
5
10
(30)
0
(20)
30
(5)
Figure 6.10 Work center load report.
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Chapter six
scheduling in A nonmAnufActuring setting
All industries deal with the issues of scheduling resources, and balancing demand and
supply as well as available versus required capacity, can be a challenge. For example,
in the transportation industry, fleets of trucks must be scheduled and routed to minimize
the total cost while ensuring timely deliveries, minimal downtime, and the nonproductive
time equated with vehicles returning empty.
In the health care industry, organizations must balance the available capacity of doctors, surgeons, nurses, technicians, operating rooms, hospital rooms, and so forth, with a
dynamic capacity required by patients, emergency vehicles, and major traumas. While
some of these may be planned, such as the scheduling of office visits and annual physicals, much of the load comes from unplanned events such as illnesses and natural disasters, and is difficult to predict. Many hospitals have begun forecasting the load by looking
at past history of patient days by month to determine trends or seasonality in order to better plan capacity. Time studies can also be done to determine standards for activities such
as lab work and surgical prep to better determine available capacity for specific resources.
The planning of resources is also critical in service industries such as retail, food, airlines,
and so forth. Scheduling of service personnel often occurs at a weekly, daily, and hourly level
based on predictions of when customers are most likely to need the service. The component of
capacity in this case is human resources, but can also include equipment, tools, and time. Some
industries, such as airlines and transportation, have to also deal with limitations of working
hours for personnel, for example, not working more than a certain number of hours in a day.
Being able to adapt the level of work force in a service industry by cross-training
employees, utilizing part-time workers, or adopting automation tools such as self-help
kiosks, a company can optimize resources and increase capacity. Nonurgent tasks, such as
cleaning and maintenance, can be performed during periods of low or no demand to utilize
personnel. One popular food chain developed its own method of scheduling that included
breaks, projections of how much food to prepare, and when to cut back on the production
of its baked goods and begin offering samples to customers.
scheduling Bottlenecks
In intermittent manufacturing, it is almost impossible to balance the available capacities of
the various workstations with the demand for their capacity. As a result, some workstations
are overloaded and some underloaded. The overloaded workstations are called bottlenecks
and, by definition, are those workstations where the required capacity is greater than the
available capacity. APICS Dictionary, 14th edition defines a bottleneck as “a facility, function, department, or resource whose capacity is less than the demand placed upon it.”
Throughput Throughput is the total volume of production passing through a facility.
Bottlenecks control the throughput of all products processed by them, as total throughput
cannot be more than can be processed through the bottleneck. If work centers feeding
bottlenecks produce more than the bottleneck can process, excess work-in-process inventory is built up. Therefore, work should be scheduled through the bottleneck at the rate it
can process the work. Work centers fed by bottlenecks have their throughput controlled by
the bottleneck, and their schedules should be determined by that of the bottleneck.
example Problem
Suppose a manufacturer makes wagons composed of a box body, a handle assembly,
and two wheel assemblies. Demand for the wagons is 500 a week. The wheel assembly capacity is 1200 sets a week, the handle assembly capacity is 450 a week, and
final assembly can produce 550 wagons a week.
a. What is the capacity of the factory?
b. What limits the throughput of the factory?
c. How many wheel assemblies should be made each week?
d. What is the utilization of the wheel assembly operation?
e. What happens if the wheel assembly utilization is increased to 100%?
Production Activity Control
145
Answer
a. 450 units a week.
b. Throughput is limited by the capacity of the handle assembly operation.
c. 900 wheel assemblies should be made each week. This matches the capacity of the
handle assembly operation.
d. Utilization of the wheel assembly operation is 900 , 1200 = 75.
e. Excess inventory builds up.
The service sector also deals with throughput, such as the length of time a patient stays at
a hospital, the number of times a restaurant turns tables during the dinner hour, or the amount
of time a customer waits in line at a bank. One of the difficulties for service organizations
is the variability in the time a service may take. The time it takes to wait on a customer at a
bank, for example, may vary considerably, depending on the number and type of transactions.
Bottleneck principles Since bottlenecks control the throughput of a facility, some
important principles should be noted:
1. Utilization of a nonbottleneck resource is not determined by its potential but by
another constraint in the process. In the previous example problem, the utilization
of the wheel assembly operation was determined by the handle assembly operation.
2. Using a nonbottleneck resource 100% of the time does not produce 100% utilization. If the wheel assembly operation was utilized 100% of the time, it would produce 1200 sets of wheels a week, 300 sets more than needed. Because of the buildup
of inventory, this operation would eventually have to stop.
3. The capacity of the facility depends on the capacity of the bottleneck. If the
handle assembly operation breaks down, the throughput of the factory is reduced.
4. Time saved at a nonbottleneck does not save capacity elsewhere. If the industrial
engineering department increased the capacity of the wheel assembly operation to 1500
units a week, the extra capacity could not be utilized, and nothing would be gained.
5. Capacity and priority must be considered together. Suppose the wagon manufacturer made wagons with two styles of handles. During setup, nothing is produced, which
reduces the capacity of the system. Since handle assembly is the bottleneck, every setup in
this operation reduces the throughput of the system. Ideally, the company would run one
style of handle for six months, then switch over to the second style. However, customers
wanting the second style of handle might not be willing to wait six months. A compromise is needed whereby runs are as long as possible but priority (demand) is satisfied.
6. Loads can, and should, be split. Suppose the handle assembly operation (the
bottleneck) produces one style of handle for two weeks, then switches to the second
style. The batch size is 900 handles. Rather than waiting until the 900 are produced
before moving them to the final assembly area, the manufacturer can move a day’s
production (90) at a time. The process batch size (900) and the transfer batch size (90)
are different. Thus, delivery to the final assembly is matched to usage, and work-inprocess inventory is reduced.
7. Focus should be on balancing the flow through the shop. The key is total throughput that ends up in saleable goods.
Managing bottlenecks Since bottlenecks are so important to the throughput of a system, scheduling and controlling them is extremely important. The following must be done:
1. Establish a time buffer before each bottleneck. A time buffer is an inventory
(queue) before each bottleneck. Because it is of the utmost importance to keep the
bottleneck working, it must never be starved for material by disrupting the flow
from feeding workstations. The time buffer should be only as long as the time of any
expected delay caused by feeding workstations. In this way, the time buffer ensures
that the bottleneck will not be shut down for lack of work and this queue will be held
at a predetermined minimum quantity.
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Chapter six
2. Control the rate of material feeding the bottleneck. A bottleneck must be fed at a
rate equal to its capacity so the time buffer remains constant. The first operation in the
sequence of operations is called a gateway operation. This operation and any other
operations prior to the bottleneck control the work feeding the bottleneck and must operate at a rate equal to the output of the bottleneck so the time buffer queue is maintained.
3. Do everything to provide the needed bottleneck capacity. Anything that increases
the capacity of the bottleneck increases the capacity of the process. Better utilization,
fewer setups, and improved methods to reduce setup and run time are some methods
for increasing capacity.
4. Adjust loads. This is similar to item 3 but puts emphasis on reducing the load on a
bottleneck by using such things as alternate work centers and subcontracting. These
may be more costly than using the bottleneck, but utilization of nonbottlenecks and
throughput of the total facility are increased, which will result in more efficient operations, and a potential for increased sales and profits.
5. Change the schedule. As discussed earlier, this should be done as a final resort, but
may be necessary in order to provide accurate delivery promises.
Once the bottleneck is scheduled according to its available capacity and the market
demand it must satisfy, the nonbottleneck resources can be scheduled. When a work order
is completed at the bottleneck, it can be scheduled on subsequent operations.
Feeding operations have to protect the time buffer by scheduling backward from the
bottleneck. If the time buffer is set at four days, the operation immediately preceding the
bottleneck is scheduled to complete the required parts four days before they are scheduled
to run on the bottleneck. Each preceding operation can be back scheduled in the same way
so the parts are available as required for the next operation.
Any disturbances in the feeding operations are absorbed by the time buffer, and
throughput is not affected. Also, work-in-process inventory is reduced. Since the queue is
limited to the time buffer, lead times are reduced.
Bottlenecks occur in every process, including hospitals, banks, and restaurants. They
must be managed, if possible, to retain customer loyalty. If the bottleneck is a result of
not enough personnel, extra hours or additional shifts may be possible. However, in some
cases, increasing capacity, such as adding extra hotel rooms or airline seats, cannot be easily accomplished.
One large airline determined that a bottleneck existed in the turnaround of an aircraft,
which was limiting the number of flights available, and causing too much downtime of their
limited resource—flight crews. From the arrival at the airport terminal, the time it took to
disembark passengers, clean out the airplane, resupply the catering items, add fuel to the
aircraft, unload and reload baggage, and get ready to board the next set of passengers, took
a minimum of 45 minutes. They determined that there was also a direct correlation between
aircraft turnaround efficiency and schedule punctuality. Through the use of technology, the
staging of luggage carts, and a more robust communication system between the flight crews
and ramp personnel, they were able to cut the time down to an average of 20 minutes. This
also had a positive impact on the on-time departure percentage, which improved by 30%.
theory of constrAints And drum-Buffer-roPe
The section on managing bottlenecks was developed based on the work of Eliyahu
M. Goldratt in his Theory of Constraints. It has allowed many people to rethink their
approaches to improving and managing their production processes. The fundamental concept behind the work is that every operation producing a product or service is a series of
linked processes. Each process has a specific capacity to produce the given defined output
for the operation, and in virtually every case, there is one process that limits or constrains
the throughput from the entire operation. Refer to Figure 6.11 for an example of an operation producing product A.
The total operation is constrained by process 3 at a capacity of four per hour. No matter how much efficiency there is in the other processes and how many process improvements are made in processes 1, 2, and 4, it will never be possible to exceed the overall
Production Activity Control
PROCESS 1
Capacity =
5 per hour
PROCESS 2
Capacity =
7 per hour
PROCESS 3
Capacity =
4 per hour
147
PROCESS 4
Capacity =
9 per hour
Figure 6.11 Product A.
operational output of four per hour. Increased efficiency and utilization in processes 1 and
2 will only increase inventory, not sales.
Identifying the constraint in a process can actually be fairly simple. There is always a
set of defined actions (processes) that are needed to create a finished product. When one
process is discovered that is working to full capacity while inventory is growing behind
the process waiting for the process, and processes downstream from that one process tend
to have idle time with respect to their need to process the inventory, then the constraint has
been identified. If all orders are scheduled and all raw material for those orders released,
yet all processes in a production sequence have idle time for the required production, then
sales is said to be the constraint.
Manage the Constraint
Several fundamental guidelines have been developed for understanding how to manage
a constraining process or bottleneck, which were discussed in the section on Bottleneck
Principles. These principles of balancing the overall flow, and maintaining steady work at
the constraint, are critical to the Theory of Constraints.
Improve the Process
Once a constraint has been identified, there is a five-step process that is recommended to
help improve the performance of the operation. The five steps are summarized as follows:
1. Identify the constraint. The entire process must be examined to determine which
process limits the throughput. The concept does not limit this process examination
to merely the operational processes. For example, in Figure 6.11, suppose the sales
department was selling the output only at the rate of three per hour. In that case, sales
would be the constraint and not process 3. It must be remembered that a constraint
limits throughput, not inventory or production.
2. Exploit the constraint. Find methods to maximize the utilization of the constraint
toward productive throughput. For example, in many operations all processes are shut
down during lunchtime. If a process is a constraint, the operation should consider
rotating lunch periods so that the constraint is never allowed to be idle.
3. Subordinate everything to the constraint. Effective utilization of the constraint is
the most important issue. Everything else is secondary.
4. Elevate the constraint. This denotes finding ways to increase the available hours of
the constraint, including getting more of it.
5. Once the constraint is no longer a constraint, find a new one and repeat the
steps. As the effective utilization of the constraint increases, it may cease to be a
constraint as another process becomes one. In that case the emphasis shifts to the new
process constraint.
Scheduling with the Theory of Constraints
The scheduling system developed for the theory of constraints (TOC) has its own specific
approach. It is often described as Drum-Buffer-Rope:
■■
Drum. The drum of the system refers to the “drumbeat” or pace of production. It represents the master schedule for the operation, which is focused around the pace of throughput as defined by the constraint. Note that once the pace of the constraint has been
defined, it does no good to schedule more production than the constraint can handle. To
do so will merely add in-process inventory and may actually decrease its effectiveness.
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Chapter six
Buffer. Since it is so important that the constraint never be “starved” for needed
inventory, a time buffer is often established in front of the constraint. It is called a time
buffer because it represents the amount of time that the inventory in the buffer protects
the constraint from disruptions. In many systems there are actually three buffers: one
for the constraint, one for assembly, and one for shipping. The constraint buffer often
represents the processing time to protect the buffer from unexpected process variation. For example, a two-day time buffer would necessitate upstream operations to
complete processing and have material in the buffer two days before actually needed.
The initial time buffer used is often the total processing time from raw material release
to reaching the constraint process. The assembly buffer often represents the time from
raw material release to a process where components that do not go through the constraint process have to be assembled with components that do have to go through the
constraint process. Finally, the buffer for shipping represents the processing time from
the point the material leaves the constraint process to completion of the final product.
■■ Rope. The analogy is that the rope “pulls” production to the constraint for necessary processing. Although this may imply a reactive replenishment system, such as a
reorder point, it can be done by a well-coordinated release of material into the system
at the right time. The rope schedules release of raw material into production at a pace
that maintains the buffer, ensures the constraint is not “starved” for material, and that
excessive inventory does not build up. It is basically defined by the processing capability of the constraint process.
■■
Even scheduling has its primary focus on effective management of the organization’s
constraint to throughput and sales.
Four primary plant types are defined, and they are used to specify the flow of materials through a production process. They can therefore be helpful in understanding how to
manage the operation using the theory of constraints. They include:
I-plant, where one raw material is used to make one final product. Processing is usually done in a straight line.
■■ A-plant, where numerous subassemblies merge into a single final assembly.
■■ V-plant, where few raw materials can be made into several end products.
■■ T-plant, where multiple straight lines can split into several assemblies.
■■
The theory of constraints also includes a process to help develop and implement
change in an organization. The first step to this is to identify core conflicts, which are then
validated by building what is called a current reality tree. After those undesirable effects
are identified from the core conflicts, a future reality tree is developed, which will lay out
a strategy to resolve the problems. The final major step in the process is to build a tactical
objective map that will define a strategy to accomplish the future reality.
example Problem
Parent X requires 1 each of component Y and Z. Both Y and Z are processed on work
center 20, which has an available capacity of 40 hours. The setup time for component Y is 1 hour and the run time 0.3 hour per piece. For component Z, setup time is
2 hours and the run time is 0.20 hour per piece. Calculate the number of Ys and Zs
that can be produced.
Answer
Let x = number of Ys and Zs to produce
TimeY + TimeZ = 40 hours
1 + 0.3x + 2 + 0.2x = 40 hours
0.5x = 37 hours
x = 74
Therefore, work center 20 can produce 74 Ys and 74 Zs.
example Problem
In this problem, parent A is made of one B and 2 Cs. As and Bs are both made on
work station 1, which has a capacity of 40 hours per week. The Cs are made on work
station 2, which also has a capacity of 40 hours per week.
Production Activity Control
Product
Setup Time (hours)
Run Time (hours/unit)
A
2
0.1
B
2
0.2
C
1
0.3
149
Based on the above information, calculate the maximum number of As, Bs, and
Cs that should be produced per week.
Answer
The number of Bs produced should equal the number of As produced to avoid over
production. Therefore, the number of Bs can be expressed in a formula as the number
of As.
Work station 1
TimeA + TimeB = 40 hours
2 + 0.1A + 2 + 0.2A = 40
0.3A = 36
A = 120
Work station 1 has the capacity to produce enough As and Bs to make 120 As
per week.
Work station 2
The number of Cs produced should be twice the number of As produced to avoid
over production. Therefore, the number of Cs can be represented by 2 * A.
TimeC = 40 Hours
1 + 2 * 0.3A = 40
0.6A = 39
A = 65
Work station 2 has the capacity to make enough Cs to support production of only
65 As per week, and in this case this is the constraint. To avoid over production, work
center 1 should produce 65 As and 65 Bs per week. Work station 2 should produce
130 Cs per week (enough for 65 As).
In this example, work station 1 will have very low utilization. However, producing
more than 65 Bs per week will only build inventory and work station 1 will be starved
by work station 2, which has the capacity to produce only 130 Cs per week.
imPlementAtion
Orders that have the necessary tooling, material, and capacity available have a good
chance of being completed on time and can be released to the shop floor. Other orders that
do not have all of the necessary elements should not be released because they only cause
excess work-in-process inventory and may interrupt work on orders that can be completed. The process for releasing an order is shown in Figure 6.12.
Implementation is performed by issuing a shop order or schedule to manufacturing,
which is the authorization for them to proceed with making the item. A shop packet that
contains the shop order and whatever other information that is needed by manufacturing
can be compiled. It may include any of the following:
Shop order showing the shop order number, part number, name, description, and quantity.
Engineering drawings.
■■ Bills of material.
■■ Route sheets showing the operations to be performed, equipment and accessories
needed, materials to use, and setup and run times.
■■ Material issue tickets that authorize manufacturing to get the required material from
stores. These are also used for charging the material against the shop order.
■■
■■
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Chapter six
REVIEW
ORDER
CHECK TOOLING
AND MATERIAL
AVAILABILITY
OKAY?
No
RESOLVE
Yes
CHECK CAPACITY
REQUIREMENTS
AND AVAILABILITY
(SCHEDULE AND
LOAD)
OKAY?
No
RESOLVE
OR
RESCHEDULE
Yes
RELEASE
ORDER
Figure 6.12 Order release process.
Tool requisitions authorizing manufacturing to withdraw necessary tooling from the
tool crib.
■■ Job tickets for each operation to be performed. As well as authorizing the individual operations to be performed, they also can function as part of a reporting system. The worker can
log on and off the job using the job ticket, and it then becomes a record of that operation.
■■ Move tickets that authorize and direct the movement of work between operations.
■■
Many manufacturing companies today use a paperless system, which authorizes production via a production schedule, rather than the release of a shop order. The shop packet
is replaced by access to the same information electronically. Online reporting of material
movement and labor reporting are used in exchange for tickets.
control
Once work orders have been issued to manufacturing, their progress has to be controlled.
To control progress, performance has to be measured and compared to what is planned. If
what is actually happening (what is measured) varies significantly from what was planned,
either the plans have to be changed or corrective action must be taken to bring performance back to plan.
The objectives of production activity control are to meet delivery dates and to make
the best use of company resources. To meet delivery dates, a company must control the
progress of orders on the shop floor, which means controlling the lead time for orders. As
discussed previously in this chapter, the largest component of lead time is queue. If queue
can be controlled, delivery dates can usually be met. Intermittent operations have many
different products and order quantities and many different routings, each requiring different capacities. In this environment, it is almost impossible to balance the load over all the
workstations. Queue exists because of this erratic input and output.
Production Activity Control
151
To control queue and meet delivery commitments, production activity control must:
Control the work going into and coming out of a work center. This is generally called
input/output control.
■■ Set the correct priority of orders to run at each work center, which is referred to as
dispatching.
■■
Input/Output Control
Production activity control must balance the flow of work to and from different work
centers. This is to ensure that queue, work-in-process, and lead times are controlled.
The input/output control system is a method of managing queues and work-in-process
lead times by monitoring and controlling the input to, and output from, a facility.
It is designed to balance the input rate in hours with the output rate so these will be
controlled.
The input rate is controlled by the release of orders to the shop floor. If the rate of
input is increased, queue, work-in-process, and lead times increase. The output rate is
controlled by increasing or decreasing the capacity of a work center. Capacity change is a
problem for manufacturing, but can be attained by overtime or undertime, shifting workers, and so forth. Figure 6.13 shows the idea graphically.
Input/output report To control input and output, a plan must be devised, along with
a method for comparing what actually occurs against what was planned. This information
is shown on an input/output report. Figure 6.14 shows an example of such a report. The
values are in standard hours.
Cumulative variance is the difference between the total planned for a given period
and the actual total for that period. It is calculated as follows:
Cumulative variance = previous cumulative variance + actual - planned
Cumulative input variance week 2 = -4 + 32 - 32 = -4
Backlog is the same as queue and expresses the work to be done in hours. It is calculated as follows:
Planned backlog for period 1 = previous backlog + planned input - planned output
= 32 + 38 - 40
= 30 hours
The report shows that the plan was to maintain a level output in each period and to
reduce the queue and lead time by 10 hours, but input and output were lower than expected.
Input Rate
Control
Queue
(Load, WIP)
Output Rate
Control
Figure 6.13 Input/output control.
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Chapter six
Work Center: 201
Capacity per Period: 40 standard hours
Period
1
2
3
4
5
Total
Planned Input
38
32
36
40
44
190
Actual Input
34
32
32
42
40
180
Cumulative Variance
–4
–4
–8
–6
–10
–10
Planned Output
40
40
40
40
40
200
Actual Output
32
36
44
44
36
192
Cumulative Variance
–8
–12
–8
–4
–8
–8
Planned Backlog
32
30
22
18
18
22
Actual Backlog
32
34
30
18
16
20
Figure 6.14 Input/output report.
Planned and actual inputs monitor the flow of work coming to the work center.
Planned and actual outputs monitor the performance of the work center. Planned and
actual backlogs monitor the queue and lead time performance.
example Problem
Complete the following input/output report for weeks 1 and 2.
Week
1
2
Planned Input
45
40
Actual Input
42
46
Planned Output
40
40
Actual Output
42
44
Cumulative Variance
Cumulative Variance
Planned Backlog
30
Actual Backlog
30
Answer
Cumulative input variance week 1 = 42 - 45 = -3
Cumulative input variance week 2 = -3 + 46 - 40 = 3
Cumulative output variance week 1 = 42 - 40 = 2
Cumulative output variance week 2 = 2 + 44 - 40 = 6
Production Activity Control
153
Planned backlog week 1 = 30 + 45 - 40 = 35
Planned backlog week 2 = 35 + 40 - 40 = 35
Actual backlog week 1 = 30 + 42 - 42 = 30
Actual backlog week 2 = 30 + 46 - 44 = 32
Operation Sequencing
APICS Dictionary, 14th edition defines operation sequencing as “a technique for shortterm planning of actual jobs to be run in each work center based on capacity (i.e., existing
workforce and machine availability) and priorities.” Priority, in this case, is the sequence
in which jobs at a work center should be worked on.
The material requirements plan establishes proper need dates and quantities for
orders. Over time, these dates and quantities change for a variety of reasons. Customers
may require different delivery quantities or dates. Deliveries of component parts, either
from suppliers or internally, may not be met. Scrap, shortages, and overages may occur. In
addition, multiple orders may have the same due date, or be scheduled to run on a particular work center the same day, but need to be sequenced. Control of priorities is exercised
through dispatching.
Dispatching Dispatching is the function of selecting and sequencing available jobs to
be run at individual work centers. The dispatch list is the instrument of priority control.
It is a listing by operation of all the jobs available to be run at a work center with the job
listed in priority sequence. It normally includes the following information and is updated
and published at least daily:
Plant, department, and work center.
Part number, shop order number, operation number, and operation description of jobs
at the work center.
■■ Standard hours.
■■ Priority information.
■■ Jobs coming to the work center.
■■
■■
Figure 6.15 shows an example of a daily dispatch list.
Dispatching rules The ranking of jobs for the dispatch list is created through the
application of dispatching or priority rules. There are many rules, some attempting to
reduce work-in-process inventory, others attempting to minimize the number of late
DISPATCH LIST
Work Center: 10
Rated Capacity: 16 standard hours per day
Shop Date: 250
Order
Number
Part
Number
Order
Quantity
123
121
142
6554
7345
2687
100
50
500
Setup Run
Hours Hours
1.5
0.5
0.2
15
30
75
Total
Quantity
Hours Completed
16.5
30.5
75.2
50
10
0
Total Available Load in Standard Hours
Jobs Coming
145
7745
135
2832
200
20
0.7
1.2
20
1.0
20.7
2.7
Total Future Load in Standard Hours
Load
Remaining
8
24
75
Operation Dates
Start
Finish
249
249
250
250
251
259
251
253
253
254
107
0
0
20.7
2.7
23.4
Figure 6.15 Dispatch list (based on 2 machines working one 8-hour shift per day).
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Chapter six
orders or maximize the output of the work center. None is perfect or will satisfy all objectives. Some commonly used rules are:
First come, first served (FCFS). Jobs are performed in the sequence in which they
are received. This rule ignores due dates and processing time.
■■ Earliest job due date (EDD). Jobs are performed according to their due dates. Due
dates are considered, but processing time is not.
■■ Earliest operation due date (ODD). Jobs are performed according to their operation due dates. Due dates and processing time are taken into account. In addition, the
operation due date is easily understood on the shop floor.
■■ Shortest process time (SPT). Jobs are sequenced according to their process time.
This rule ignores due dates, but it maximizes the number of jobs processed. Orders
with long process times tend to be delayed.
■■
Figure 6.16 illustrates how these sequencing rules work. Notice that each rule usually
produces a different sequence.
Critical ratio (CR). Critical ratio considers due dates and process time, and is an
index of the relative priority of an order to other orders at a work center. It is based on the
ratio of time remaining to work remaining and is expressed as:
CR =
due date - present date
actual time remaining
=
lead time remaining
lead time remaining
Lead time remaining includes all elements of manufacturing lead time that have not
yet been processed and expresses the amount of time the job normally takes to complete.
If the actual time remaining is less than the lead time remaining, it implies there is not
sufficient time to complete the job and the job is behind schedule. Similarly, if lead time
remaining and actual time remaining are the same, the job is on schedule. If the actual
time remaining is greater than the lead time remaining, the job is ahead of schedule. If the
actual time remaining is less than 1, the job is late already. The following table summarizes these facts and relates them to the critical ratio:
CR less than 1 (actual time less than lead time).
CR equal to 1 (actual time equal to lead time).
CR greater than 1 (actual time greater than lead time).
CR zero or less (today’s date greater than due date).
Order is behind schedule.
Order is on schedule.
Order is ahead of schedule.
Order is already late.
Using the critical ratio dispatching rule, orders are listed in order of their critical
ratio with the lowest one first. The critical ratio of an order may change as the actual time
remaining and lead time remaining change.
Sequencing Rule
Job
Process
Time (days)
Arrival
Date
Due
Date
Operation
Due Date
FCFS
EDD
ODD
SPT
A
4
223
245
233
2
4
1
3
B
1
224
242
239
3
2
2
1
C
5
231
240
240
4
1
3
4
D
2
219
243
242
1
3
4
2
Figure 6.16 Application of sequencing rules.
Production Activity Control
155
example Problem
Today’s date is 175. Orders A, B, and C have the following due dates and lead time
remaining. Calculate the actual time remaining and the critical ratio for each.
Order
Lead Time
Remaining (days)
Due Date
A
185
20
B
195
20
C
205
20
Answer
Order A has a due date of 185, and today is day 175. There are 10 actual days remaining. Since the lead time remaining is 20 days,
Critical ratio =
10
= 0.5
20
Similarly, the actual time remaining and the critical ratios are calculated for orders
B and C. The following table gives the results:
Order
Due Date
Lead Time
Remaining (days)
Actual Time
Remaining (days)
CR
A
185
20
10
0.5
B
195
20
20
1.0
C
205
20
30
1.5
Order A has less actual time remaining than lead time remaining, so the CR is
less than 1. It is, therefore, behind schedule. Order B has a CR of 1 and is exactly on
schedule. Order C has a CR of 1.5—greater than 1—and is ahead of schedule.
An additional principle sometimes used for sequencing is slack time. Slack time is the
result of adding up the remaining setup and run times for an order, and subtracting that
from the time remaining. The job with the least slack would be scheduled first. Slack time
can also be divided by the number of remaining operations, which is called slack per
operation, where the job with the smallest value would be the priority.
Dispatching rules should be simple to use and easy to understand. As shown in Figure
6.16, each rule produces a different sequence and has its own advantages and disadvantages.
Whichever rule is selected, it should be consistent with the objectives of the planning system.
Production rePorting
Production reporting provides feedback of what is actually happening on the plant floor. It
allows PAC to maintain valid records of on-hand and on-order balances, job status, shortages, scrap, material shortages, and so on. Production activity control needs this information to establish proper priorities and to answer questions regarding deliveries, shortages,
and the status of orders. Manufacturing management needs this information to make decisions about plant operations. Payroll needs this information to calculate employees’ pay.
Data must be collected, sorted, and reported. The particular data collected depends upon
the needs of the various departments. The methods of data collection vary. Sometimes the
operator reports the start and completion of an operation, order, movement, and so on, using
an online system directly reporting events as they occur. In other cases, the operator, supervisor, or timekeeper reports this information on an operation reporting form included in the
shop packet. Information about inventory withdrawals and receipts must be reported as well.
Once the data is collected and recorded, appropriate reports are produced. Types of
information needed for the various reports include:
■■
■■
Order status.
Weekly input/output by department or work center.
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Chapter six
Exception reports on such things as scrap, rework, and late shop orders.
Inventory status.
■■ Performance summaries on order status, work center and department efficiencies, and so on.
■■
■■
Product trAcking
Production control is often responsible for product tracking or lot traceability. This is
the process of tracking parts and materials back to their origins. It has a very practical
application in the matching of colors in fabrics and paint ensuring the consumer that
different units of the product match in color at the time of manufacturing and during the
product’s lifetime. Traceability may also be legislated in industries such as food, pharmaceutical, or aerospace to ensure the safety of the product. Should a product prove unsafe,
it is possible for the manufacturer to trace back to find the source of all materials and
recall all finished products that used that particular lot. This is a very meticulous process
with information collected along with other supply chain information, often by production activity control personnel.
meAsurement systems
As mentioned previously, to control progress, as well as adjust plans, performance has to
be measured and compared to what is planned. In the area of production activity control,
there are many types of performance measurement systems available. The primary purpose of these measurements is to provide an objective means of evaluating performance,
and to take corrective action if necessary. It is also important to make sure whatever type
of measurement is used, it is aligned with the overall performance measurement of the
organization.
In addition to those already discussed in this text, such as utilization, efficiency,
productivity, demonstrated capacity, and input/output control, some of the more common
measurements used are as follows:
Actual vs. planned lead time: A comparison of the actual throughput time to the
stated lead time.
Percent orders completed on time: Percentage of orders completed on the due date,
rather than early or late
Performance to schedule: A measure of the quantity and date produced as compared
to the master schedule.
Measurement systems will be discussed further in Chapters 15 and 16.
summAry
Production activity control is concerned with converting the material requirements plan
into action, reporting the results achieved, and when required, revising the plans and
actions to meet the required results. Order release, dispatching, and progress reporting are
the three primary functions. To accomplish the plans, PAC must establish detailed schedules for each order, set priorities for work to be done at each work center, and keep them
current. Production activity control is also responsible for managing the queue and lead
times. Nonmanufacturing industries must also control capacity and inventory in order to
monitor progress, manage resources, and derive appropriate schedules.
The theory of constraints modifies the approach to PAC since it views a production facility, the suppliers, and the market as a series of interdependent functions. TOC
attempts to optimize the constraints (bottlenecks) in a system as they affect the overall
throughput. As a result, traditional lot sizing rules should be modified to increase the
throughput of the entire process and not just the individual work centers. Drum-bufferrope describes how the TOC works by setting an overall pace of material flow, ensuring
Production Activity Control
157
bottlenecks never run out of material, and linking the output of one work center to another.
Measurement systems are used by PAC to monitor and control progress, meet delivery
dates, utilize labor and equipment efficiently, and keep inventory levels down.
key terms
Backward scheduling 140
Bill of material 136
Bottlenecks 144
Continuous manufacturing 135
Critical ratio (CR) 154
Cumulative variance 151
Cycle time 139
Dispatching 153
Drum-Buffer-Rope 147
Earliest job due date 154
Earliest operation due date 154
Finite loading 140
Manufacturing lead time 138
Move time 138
Operation overlapping 141
Operation splitting 142
Process batch 141
Product structure 136
Product tracking 156
Project manufacturing 136
Queue time 138
Repetitive manufacturing 135
Routing 136
Run time 138
First come, first served 154
Flow manufacturing 135
Forward scheduling 139
Gateway operation 146
Infinite loading 140
Input/output control 151
Input/output report 151
Intermittent manufacturing 135
Item master 136
Load profile 143
Setup time 138
Shop order master 137
Shortest process time 154
Slack per operation 155
Slack time 155
Throughput 144
Throughput time 139
Transfer batch 141
Wait time 138
Work center master 137
questions
1. What is the responsibility of production activity control?
2. What are the major functions of planning, implementation, and control?
3. What are the major characteristics of flow, intermittent, and project manufacturing?
4. Why is production activity control more complex in intermittent manufacturing?
5. To plan the flow of materials through manufacturing, what four things must production activity
control know? Where will information on each be obtained?
6. What are the four types of planning data used in production activity control? What information
does each contain?
7. What information is used for controlling production?
8. What should production activity control check before releasing a shop order?
9. What is manufacturing lead time? Name and describe each of its elements.
10. Describe forward and backward scheduling. Why is backward scheduling preferred?
11. Describe infinite and finite loading.
12. What is operation overlapping? What is its purpose?
13. What is operation splitting? What is its purpose?
14. What information does a load report contain? Why is it useful to production activity control?
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Chapter six
15. What is a bottleneck operation?
16. What is the definition of throughput?
17. What are the seven bottleneck principles discussed in the text?
18. What are the five things discussed in the text that are important in managing bottlenecks?
19. What is a shop order? What kind of information does it usually contain?
20. What two things must be done to control queue and meet delivery commitments?
21. What is an input/output control system designed to do? How is input controlled? How is output
controlled?
22. What is dispatching? What is a dispatch list?
23. Describe each of the following dispatching rules giving their advantages and disadvantages.
a. First come, first served.
b. Earliest due date.
c. Earliest operation due date.
d. Shortest processing time.
e. Critical ratio.
24. If the time remaining to complete a job is 10 days and the lead time remaining is 12 days, what
is the critical ratio? Is the order ahead of schedule, on schedule, or behind schedule?
25. Would critical ratio be better utilized as a static ratio or a dynamic ratio, and why?
26. What is the purpose of production reporting? Why is it needed?
27. A student of production inventory management has decided to apply critical ratio to his homework assignments. Describe what is happening if his critical ratio for various assignments is:
a. negative.
b. zero.
c. between zero and 1.
d. greater than 1.
28. Bottlenecks exist in many business processes that serve the public and are usually indicated by
lineups. Choose a business that experiences lineups and identify the constraints in the system.
Give specific examples of each of the seven bottleneck principles that apply to that business.
Suggest a way to increase the throughput of the bottleneck and describe the benefits to the business and to the customers.
29. What is lot traceability? Why is it important to safety related products?
30. Choose a service industry and describe the scheduling and bottleneck issues that must be controlled in order to maintain customer service.
31. Provide an explanation of Drum-Buffer-Rope and give an example of how it would be used.
ProBlems
6.1. Shop order 7777 is for 600 of part 8900. From the routing file, it is found that operation 20 is done on work center 300. The setup time is 3.5 hours, and run time is 0.233
hours per piece. What is the required capacity on work center 300 for shop order 7777?
Answer.
143.3 standard hours
6.2. An order for 100 of a product is processed on work centers A and B. The setup time on
A is 50 minutes, and run time is 5 minutes per piece. The setup time on B is 60 minutes, and the run time is 5 minutes per piece. Wait time between the two operations is
5 hours. The move time between A and B is 40 minutes. Wait time after operation B is
5 hours, and the move time into stores is 3 hours. Queue at work center A is 25 hours
and at B is 35 hours. Calculate the total manufacturing lead time for the order.
Answer.
92 hours and 10 minutes
6.3. In problem 6.2, what percentage of the time is the order actually running?
Answer.
18.08%
6.4. An order for 50 of a product is processed on work centers A and B. The setup time on
A is 45 minutes, and run time is 5 minutes per piece. The setup time on B is 30 minutes,
Production Activity Control
159
and the run time is 4 minutes per piece. Wait time between the two operations is
8 hours. The move time between A and B is 60 minutes. Wait time after operation B is
8 hours, and the move time into stores is 2 hours. Queue at work center A is 40 hours
and at B is 35 hours. Calculate the total manufacturing lead time for the order.
6.5. In problem 6.4, what percentage of time is the order actually running?
6.6. Amalgamated Skyhooks, Inc., has an order for 200 Model SKY3 Skyhooks for
delivery on day 200. The Skyhook consists of three parts. Components B and C form
subassembly A. Subassembly A and component D form the final assembly. Following
are the work centers and times for each operation. Using a piece of graph paper, draw
a backward schedule based on the following. When must component C be started to
meet the delivery date?
a. Only one machine is assigned to each operation.
b. The factory works one 8-hour shift, 5 days a week.
c. All parts move in one lot of 200.
Part
Operation
Standard Time (days)
D
10
5
20
7
10
5
B
C
Answer.
20
7
10
12
20
5
Subassembly A
7
Final Assembly SKY3
5
Day 171
6.7. International Door Slammers has an order to deliver 500 door slammers on day 130.
Draw up a backward schedule under the following conditions:
a. Only one machine is assigned to each operation.
b. Schedule one 8-hour shift per day for 5 days per week.
c. All parts are to move in one lot of 500 pieces.
d. Allow 8 hours between operations for queue and move times.
A slammer consists of three parts. Purchased components C and D form subassembly A. Subassembly A and component B form the final assembly. Part B is
machined in three operations. No special tooling is required except for part B, operation 20. It takes 24 hours to make the tooling. Material is available for all parts.
Standard times for the lot of 500 are as follows:
Part
Operation
Standard Time (days)
B
10
10
20
8
30
6
Subassembly A
18
Final assembly
5
6.8. An order for 100 of a product is processed on operation A and operation B. The setup
time on A is 50 minutes, and the run time per piece is 9 minutes. The setup time on
B is 30 minutes, and the run time is 6 minutes per piece. It takes 20 minutes to move
a lot between A and B. Since this is a rush order, it is given top priority (president’s
edict) and is run as soon as it arrives at either workstation.
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Chapter six
It is decided to overlap the two operations and to split the lot of 100 into two lots
of 60 and 40. When the first lot is finished on operation A, it is moved to operation B
where it is set up and run. Meanwhile, operation A completes the balance of the 100
units (40) and sends the units over to operation B. These 40 units should arrive as
operation B is completing the first batch of 60; thus, operation B can continue without
interruption until all 100 are completed.
a. Calculate the total manufacturing lead time for operation A and for B without
overlapping.
b. Calculate the manufacturing lead time if the operations are overlapped. How much
time is saved?
Answer.
a. Total manufacturing lead time = 1600 minutes
b. Total manufacturing lead time = 1240 minutes
Saving in lead time = 360 minutes
6.9. An order for 250 bell ringers is processed on work centers 10 and 20. The setup and
run times are as follows. It is decided to overlap the lot on the two work centers and
to split the lot into two lots of 100 and 150. Move time between operations is 30 minutes. Work center 20 cannot be set up until the first lot arrives. Calculate the saving in
manufacturing lead time.
Setup on A = 50 minutes
Run time on A = 5 minutes per piece
Setup on B = 100 minutes
Run time on B = 7 minutes per piece
6.10. An order for 100 of a product is processed on operation A. The setup time is 50 minutes, and the run time per piece is 9 minutes. Since this is a rush order, it is to be split
into two lots of 50 each and run on two machines in the work center. The machines
can be set up simultaneously.
a. Calculate the manufacturing lead time if the 100 units are run on one machine.
b. Calculate the manufacturing lead time when run on two machines simultaneously.
c. Calculate the reduction in lead time.
Answer.
a. 950 minutes
b. 500 minutes
c. 450 minutes
6.11. What would be the reduction in manufacturing lead time if the second machine
could not be set up until the setup was completed on the first machine?
6.12. An order for 100 of a product is run on work center 40. The setup time is 4 hours,
and the run time is 3 minutes per piece. Since the order is a rush and there are
two machines in the work center, it is decided to split the order and run it on both
machines. Calculate the manufacturing lead time before and after splitting.
6.13. In problem 6.12, what would be the manufacturing lead time if the second machine
could not be set up until the setup on the first machine was completed? Would there
be any reduction in manufacturing lead time?
6.14. Complete the following input/output report. What are the planned and actual backlogs at the end of period?
Period
1
2
3
4
Planned Input
35
38
36
39
Actual Input
33
33
31
40
Cumulative Variance
Total
Production Activity Control
Planned Output
40
40
40
40
Actual Output
38
35
40
38
161
Cumulative Variance
Planned Backlog
32
Actual Backlog
32
Answer.
Planned backlog = 20 units. Actual backlog = 18 units
6.15. Complete the following input/output report. What is the actual backlog at the end of
period 5?
Period
1
2
3
4
5
Planned Input
78
78
78
78
78
Actual Input
84
80
78
82
80
Planned Output
80
80
80
80
80
Actual Output
85
83
76
82
84
Total
Cumulative Variance
Cumulative Variance
Planned Backlog
45
Actual Backlog
45
6.16. Complete the following table to determine the run sequence for each of the sequencing rules.
Sequencing Rule
Job
Process
Time (days)
Arrival
Date
Due
Date
Operation
Due Date
A
5
123
142
132
B
2
124
144
131
C
3
131
140
129
D
6
132
146
135
FCFS
EDD
ODD
SPT
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Chapter six
6.17. Jobs A, B, and C are in queue at work center 10 before being completed on work
center 20. The following information pertains to the jobs and the work centers. For
this problem, there is no move time. Today is day 1. If the jobs are scheduled by the
earliest due date, can they be completed on time?
Job
Process Time (days)
Due Date
Work Center 10
Work Center 20
A
7
3
12
B
5
2
24
C
9
4
18
Job
Work Center 10
Start Day
Stop Day
Work Center 20
Start Day
Stop Day
A
C
B
6.18. Calculate the critical ratios for the following orders and establish in what order they
should be run. Today’s date is 75.
Order
Due Date
Lead Time
Remaining (days)
A
89
10
B
95
25
C
100
20
Actual Time
Remaining (days)
CR
cAse study 6.1
Johnston Products
No matter how many times Justin Wang, the master scheduler for Johnston Products,
tried, he couldn’t seem to get it through people’s minds. They kept trying to “front
load” the production schedule by attempting to catch up with production they failed to
make the previous week, and the problem appeared to be getting worse. It seemed to
happen every week, and the only way Justin could get things back to a realistic position
was to completely reconstruct the entire master schedule—usually about every three
weeks.
Last month could serve as an example. The first week of the month Justin had scheduled production equal to 320 standard hours in the assembly area. The assembly area managed to complete only 291 hours that week because of some equipment maintenance and
a few unexpected part shortages. The assembly supervisor then had the workers complete
the remaining 29 hours from week 1 at the start of week 2. Since week 2 already had 330
standard hours scheduled, the additional 29 hours really put them in a position of attempting to complete 359 hours. The workers actually completed 302 hours in week 2, leaving
Production Activity Control
163
57 hours to front load into week 3, and so forth. Usually by the time Justin came to his
three-week review of the master schedule, it was not uncommon for the assembly area to
be more than 100 standard hours behind schedule.
Clearly, something needed to be done. Justin decided to review some of the areas that
could be causing the problem:
1. Job standards Although it had been at least four years since any job standards had
been reviewed or changed, Jason felt the standards could not be the problem—quite
the opposite. His operations course had taught him about the concept of the learning
curve, implying that if anything the standard times for the jobs should be too high,
allowing the average worker to complete even more production per hour than that
implied by the job standard.
2. Utilization The general manager was very insistent on high utilization of the area.
He felt that it would help control costs, and consequently used utilization as a major
performance measure for the assembly area. The problem was that customer service
was also extremely important. With the problems Justin was having with the master
schedule, it was difficult to promise order delivery accurately, and equally difficult to
deliver the product on time once the order promise was made.
3. The workers In an effort to control costs, the hourly wage for the workers was not
very high. This caused a turnover in the workforce of almost 70% per year. In spite
of this, the facility was located in an area where replacement workers were fairly
easy to hire. They were assigned to the production area after they had a minimum of
one week’s worth of training on the equipment. In the meantime, the company filled
vacant positions with temporary workers brought in by a local temporary employment
service.
4. Engineering changes The design of virtually all the products was changing,
with the average product changing with respect to some aspect of the design
about every two months. Usually this resulted in an improvement to the products, however, so Justin quickly dismissed the changes as a problem. There were
also some engineering changes on the equipment, but in general little in the way
of process change had been made. The setup time for a batch of a specific design
had remained at about 15 minutes. That forced a batch size of about from 50 to
300 units, depending on the design. The equipment was getting rather old, however, forcing regular maintenance as well as causing an occasional breakdown.
Each piece of equipment generally required about three hours of maintenance
per week.
Since the computer had done most of his calculations in the past, Justin decided to
check to see if the computer was the source of the problem. He gathered information to
conduct a manual calculation on a week when there were eight people assigned to the assembly area (one person for each of eight machines) for one shift per day. With no overtime, that would allow 320 hours of production.
Product
A174
G820
H221
B327
C803
P932
F732
J513
L683
Batch Size
50
100
50
200
100
300
200
150
150
Standard Assembly Time
(minutes per item)
17
9
19.5
11.7
21.2
14.1
15.8
17.3
12.8
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Chapter six
assignment
1. With this information, Justin calculated the total standard time required to be within
the 320 hours available. Is he correct? Calculate the time required and check the accuracy of his calculation.
2. List the areas you think are causing trouble in this facility.
3. Develop a plan to deal with the situation and try to get the production schedule back
under control under the constraints listed.
cAse study 6.2
Crofts Printing Company
John Burton was not a happy man. He was a supervisor for the Crofts Printing Company, having been recently promoted from lead printer. While he felt very comfortable with his knowledge and success in the printing business, this managerial position was starting to wear on him.
He was determined not to let it get him down, however, as he felt he surely had the knowledge,
experience, and respect of the workers. He had been asking Jason Crofts for months for a
chance at management, and he certainly wasn’t about to let it get the better of him.
His current problem had to do with scheduling. Since he had become supervisor the
sales people always asked him about an order before they promised delivery to a customer.
He thought that would be quite simple; after all, who knew more about the printing business
than he did? Based on his knowledge of the processes and what was already in progress,
he gave what he thought were reasonable, even conservative, estimates of promise dates.
Unfortunately, his track record was not too good. There had been many late deliveries since
his managerial appointment, and nobody in the organization was too happy about it.
At first he thought it must be the other workers. “They’re just jealous about my selection as supervisor and want to make me look bad” was his initial reaction. Henry Hurley,
another long-time machine operator, was John’s best friend. One afternoon over a beer,
John asked Henry about the problem in a confidential discussion. Henry said he was sure
that John had been trying to get it right but somehow it didn’t seem to be going well.
Henry assured John that the workers were trying their best. In fact, according to Henry,
the workers had been putting in extra effort. They viewed John’s promotion as a positive
sign that there was a possible future for them in management as well. John’s failure would
have been, in fact, greatly discouraging to most of the workers.
John then thought he might be the problem when it came to giving estimates. The sales
people would almost always contact him about a possible job to ask him when it should be
promised to the customer. His great knowledge of the printing business allowed him, he
thought, to quickly come to a good estimate. Perhaps he was not as good at estimating as
he thought. To check this out, he looked at most of the jobs done during the last couple of
weeks. In almost every case, the work recorded against a job was almost exactly what he
had estimated. What little error existed was certainly not large enough to cause the problem.
John trusted Henry and believed his account of the situation, and his analysis of the
estimates convinced him the problem wasn’t there. If it wasn’t the workers and wasn’t the
estimates then what could it be? He must do something. Jason Crofts was a patient man,
but there was a limit. He was worried about alienating his best customers, and at the same
time knew he must be concerned about efficiency as a way to control cost.
John decided there was a need to take drastic action to ease the situation, or at least to
find out what the cause was. On a Friday he scheduled overtime for Saturday to finish all
jobs in progress. On Monday, therefore, he could start with a clean slate. There were several jobs already promised, but not yet started. He figured that on Monday he could start
with all new jobs and really figure out the source of his problem.
The jobs were all promised within four days, but he figured there should be no problem. He had three operations, and most of the jobs went through all three, but not all jobs
needed all operations. He had one worker assigned to each operation. Over the next four
days that represented 96 hours of available work time (3 operations * 4 days * 8 hours
Production Activity Control
165
per day), he had eight jobs promised. The total estimated time for all eight jobs was only
88 hours, giving him a buffer of 8 hours over the next four days. To make sure there would
be no problem, he decided there would be no new jobs even scheduled to start during
those four days, with the only exception being if any operation completed all the necessary work for all eight jobs before the end of the four days, they could start another. In any
case, he wanted to make sure that if necessary, all 96 hours would be reserved for just the
88 hours of scheduled work.
John had learned that a good priority rule to use was the critical ratio rule, primarily
because it took into account both the customer due date and the amount of processing time
for a job. He therefore used that rule to prioritize the jobs. The following table shows the
eight jobs, together with processing time estimates and due dates. All due dates are at the
end of the day indicated. Processing times for all jobs at all three operations are in hours.
Job
Operation 1 Operation 2 Operation 3
Total Time
Day Due
A
5 hours
3 hours
4 hours
12 hours
Tuesday
B
0
6
2
8
Wednesday
C
4
2
5
11
Tuesday
D
7
0
3
10
Thursday
E
2
8
0
10
Thursday
F
0
6
3
9
Thursday
G
3
3
4
10
Thursday
H
6
5
6
17
Thursday
assignment
1. Using the critical ratio rule, establish the priority for the eight jobs.
2. Use a chart to load the operation according to the priority rule established. In other
words, load the most important job in all three work centers, then the next most
important, and so forth. This is the method that John used.
3. Analyze John’s approach and try to determine if he has a problem, and if he does
determine the source of the problem.
4. Try to provide a solution to John that will ease the problem, and perhaps eliminate it.
cAse study 6.3
Melrose Products
Jim Hartough was not in a good mood. He worked his way through the ranks when supervisors did supervision and workers did what they were told. He was now faced with the fact
that the new president of Melrose products was one of these “touchy-feely” types that was
pushing for self-directed work teams. As the manufacturing manager, Jim was ultimately
responsible to not only meet production needs, but also to do so in the most efficient and
cost-effective manner possible. To him, that meant specific allocation of work. It had always worked that way and he saw nothing new to tell him it shouldn’t continue to do so.
Part of the problem, Jim realized, was that the business environment was changing.
Changes in the product design were becoming more frequent and the customers were
expecting more service. While they were still sensitive to price (the competition had
not disappeared), they wanted quick delivery, high quality, and the product designed
more specifically to their need. To Jim, that meant putting more pressure on those
“lazy, pampered engineers” to make better designs as well as additional pressure on
those “bums on the factory floor” to meet production needs. With better designs, he
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Chapter six
could more easily allocate the work to his workforce to meet the customer demands.
He felt he had truly kept up with the times—the customer was king. The fact that the
customer expected more meant little more than how to get them what they wanted from
production. It was merely a case of making sure everyone delivered on the job the way
they were supposed to.
While Mr. Melrose had avoided the need to become a public company and had
managed to keep unions out, he had still apparently gone soft, at least according to
Jim. He had recently appointed Cindy Lopez as the new president, passing over Jim.
She not only had an MBA (Jim had always thought the real business learning was done
“on the firing line”), but also had never even been a supervisor. She had come from, of
all places, the human resources department! That department had never done anything
for him other than send him a bunch of worthless people. Some of those people had, in
his mind, no chance of ever becoming useful. As far as he was concerned, the only real
value of a human resources department was to keep the government idiotic bureaucrats
off their backs.
So, now Jim was in the position to try to “change with the times,” as Cindy had
said. She wanted to gradually move the company toward flexible self-directed work
teams. Jim, of course, felt that all the workers really wanted was to get their paycheck
and party on Friday and Saturday nights, and could care less about having any say in
the product or the customer. How was he ever going to get anything done with someone so naive in charge?
the Current situation
Cindy had suggested that Jim start the process of changing to teams by looking at the
K-line. The K-line of product was a fairly standard product that had recently undergone
heavy competitive pressure in the form of delivery speed and design enhancements.
Melrose had been gradually losing market share in the K-line. Jim had responded, before
the naming of Cindy as president, by putting additional pressure on workers to be more efficient and reducing their task times. As Jim said, “there’s always some slack time we can
squeeze out of any process if we really put our minds to it.”
They are using carefully developed time standards, much as Jim learned in his
Industrial Engineering courses. He feels they are quite good, including a liberal 10%
allowance. Since the K-line is a fairly standard product, Jim not only uses the time
standard to develop cost figures for labor, but also uses those cost figures to allocate
overhead.
There are currently seven labor tasks to make one of the K-line products.
Task
Standard Time (Min.)
Estimated Labor Cost/Minute
1
7.5
$0.24
2
2.3
$0.22
3
4.7
$0.28
4
5.1
$0.29
5
17.8
$0.26
6
19.1
$0.18
7
8.4
$0.25
The overhead allocation is currently at 230% of direct labor. Material costs are $9.35
per unit. They currently have enough labor to produce 20 of the K-line per shift. Each shift
has one supervisor costing about $24 per hour, accounted for in the overhead account.
From this information, Jim was being asked to develop teams, and without direct
supervision. From his standpoint, the effort was doomed to failure. Jim, however, always
considered himself a “company man” and would do what he could to make it happen.
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assignment
1. What is the standard cost of the K-line product?
2. What specific steps would you undertake to make the self-directed teams? How, specifically, would you deal with the cost and time standard issues?
3. Do you agree with Cindy? Do you agree with Jim? Is there some other alternative
approach that might be better in this situation? Explain.
4. What do you do with the supervisor in this situation? Be specific in your approach.
5. How do you deal with Jim? Develop a specific plan to deal with a situation such as
the one described.
6. Are self-directed work teams the answer? Where should or shouldn’t they be used.
Discuss the pros and cons of such teams and where, or where not, they should be
used, and how they would be used in this situation, if appropriate.
Chapter
seven
PurchasIng
IntroductIon
Purchasing can simply be considered the process of buying. Many assume purchasing is
solely the responsibility of the purchasing department. However, the function is much
broader and, if carried out effectively, all departments in the company may be involved.
Obtaining the right material, in the right quantities, with the right delivery (time and
place), from the right source, and at the right price are all purchasing functions.
Choosing the right material requires input from the marketing, engineering, manufacturing, and purchasing departments. Quantities and delivery of finished goods are
established by the needs of the marketplace. However, manufacturing planning and control must decide when to order which raw materials so that marketplace demands can be
satisfied. Purchasing is then responsible for placing the orders and for ensuring that the
goods arrive on time.
The purchasing department has the major responsibility for locating suitable sources
of supply and for negotiating prices. Input from other departments may be required in
finding and evaluating sources of supply and to help the purchasing department in price
negotiation. Environmental responsibility is becoming a major consideration in business
due to potential costs and consumer demand. Purchasing departments are in a position to
take the lead role in reducing a company’s environmental impact since they are familiar
with all materials purchased and have excellent contacts with suppliers for product information. Purchasing, in its broad sense, is everyone’s business.
Purchasing and Profit Leverage
On the average, manufacturing firms spend about 50% of their sales dollars in the purchase of raw materials, components, and supplies. This gives the purchasing function
tremendous potential to reduce costs and increase profits. As a simple example, suppose a
firm spends 50% of its revenue on purchased goods and shows a net profit before taxes of
10%. For every $100 of sales, it receives $10 of profit and spends $50 on purchases. Other
expenses are $40. For the moment, assume that all costs vary with sales. These figures are
shown in the following as a simplified income statement:
InCOME STATEMEnT
Sales
Cost of Goods Sold
Purchases
Other Expenses
Profit Before Tax
$100
$50
40
90
$10
To increase profits by $1, a 10% increase in profits, sales must be increased to $110.
Purchases and other expenses increase to $55 and $44. The following modified income
statement shows these figures:
168
Purchasing
169
InCOME STATEMEnT (SAlES InCrEASE)
Sales
$110
Cost of Goods Sold
Purchases
$55
99
Other Expenses
44
Profit Before Tax
$11
However, if the firm can reduce the cost of purchases from $50 to $49, a 2% reduction, it would gain the same 10% increase in profits. In this particular example, a 2%
reduction in purchase cost has the same impact on profit as a 10% increase in sales.
InCOME STATEMEnT (rEduCEd PurCHASE COST)
Sales
$100
Cost of Goods Sold
Purchases
$49
Other Expenses
40
89
Profit Before Tax
$11
Purchasing Objectives
Purchasing is responsible for the flow of materials into the firm, following up with the
supplier, and expediting delivery. Missed deliveries can create havoc for manufacturing
and sales, and purchasing can reduce problems for both areas, further adding to the profit.
The objectives of purchasing can be divided into five categories:
Obtaining goods and services of the required quantity and quality.
■■ Obtaining goods and services at the lowest total cost.
■■ Ensuring the best possible service and prompt delivery by the supplier.
■■ developing and maintaining good supplier relations and developing potential suppliers.
■■ Selecting products and suppliers that minimize the impact on the environment.
■■
To satisfy these objectives, some basic functions must be performed:
determining purchasing specifications: right quality, right quantity, and right delivery
(time and place).
■■ Selecting supplier (right source).
■■ negotiating terms and conditions of purchase (right price).
■■ Issuing and administration of purchase orders and agreements.
■■
Outsourcing
APICS Dictionary, 14th edition defines outsourcing as “the process of having suppliers provide goods and services that were previously provided internally.” One method of outsourcing is offshoring, which is defined as “outsourcing a business function to another company in
a different country than the original company’s country.” This is a growing trend with many
companies as lower labor costs and an increasingly educated workforce are becoming available offshore. Internet communications and efficient shipping methods can make outsourcing
and offshore sourcing very attractive. Companies are under pressure to reduce costs and to
focus on their core competencies, which can turn outsourcing into a competitive advantage.
The purchasing department is directly affected by the growth in outsourcing. Operations
departments add value to goods through efficiently using people, machines, and materials.
Many of these components are now purchased rather than produced internally, and there
is a shift from internal management of staff to working with outside suppliers through the
purchasing department. The outsourcing trend is not just occurring in manufacturing but is
also affecting service departments as companies may outsource maintenance, information
technology, logistics, finance, and customer service. Outside contractors can often do the
same work better, faster, and cheaper.
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Purchasing departments have an increasing responsibility with the management of
outside operations and the development and administration of contracts. Contracts and
legal terms are beyond the scope of this text.
Purchasing Cycle
The purchasing cycle consists of the following steps:
1. receiving and analyzing purchase requisitions.
2. Selecting suppliers, including researching and finding potential suppliers, issuing requests
for quotations, receiving and analyzing quotations, and selecting the right supplier.
3. determining the right price.
4. Issuing purchase orders and agreements.
5. Following up to ensure delivery dates are met.
6. receiving and accepting goods.
7. Approving supplier’s invoice for payment.
Receiving and analyzing purchase requisitions
Purchase requisitions start with
the department or person who will be the ultimate user. In the material requirements planning (MrP) environment, the planner or buyer/planner releases a planned order authorizing
the purchasing department to go ahead and process a purchase order. For items not used in
the manufacturing process, such as maintenance, repair, and operating (MRO) items, office supplies, or capital equipment, a paper or electronic requisition is sent to the purchasing
department. At a minimum, the purchase requisition contains the following information:
Identity of originator, signed approval, and account to which cost is assigned.
Material specification.
■■ Quantity and unit of measure.
■■ required delivery date and place.
■■ Any other supplemental information needed.
■■
■■
Electronic requisition systems are now widely used and are often part of enterprise resource planning (ERP) software. The minimum requisition information is still
required, and the system can supply much of the details and control of the information
based on predetermined settings. For example, the requisitioner can enter the desired part
number and ErP will provide the appropriate description, specification, suggested suppliers, shipping instructions, and so on. The system will then forward the requisition for
the appropriate approvals with controls in place for account number and spending limits.
Once all the approvals have been completed, the requisition is sent to the purchasing
department to produce the purchase order without reentering all the information. For items
of small value (see C items covered in Chapter 9) that are ordered frequently, the system
may send an electronic release of material directly to the approved supplier. The benefits
of these tools to the company are ease of entry for the requisitioner, reduced paperwork,
decreased turnaround time of requisitions, and improved accuracy of information.
Selecting suppliers Identifying and selecting suppliers are important responsibilities
of the purchasing department. For routine items or those that have been purchased before,
a list of approved suppliers is maintained. If the item has not been purchased before or
there is no acceptable known supplier, a search must be made. Other items may require
input from the engineering and design departments for suggested suppliers. More on
selecting suppliers will be discussed later in this chapter.
Requesting quotations For high cost items, it is usually desirable to issue a request for
quote (RFQ). This is a written inquiry that is sent to enough suppliers to be sure competitive
and reliable quotations are received. It is not a purchase order. After the suppliers have completed and returned the quotations to the buyer, the quotations are analyzed for price, total
cost, compliance to specification, terms and conditions of sale, delivery, and payment terms.
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171
Determining the right price This is the responsibility of the purchasing department
and is closely tied to the selection of suppliers. The purchasing department is also responsible for price negotiation and will try to obtain the best price from the supplier. Price
negotiation will be discussed in a later section of this chapter.
Issuing a purchase order A purchase order is a legal offer to purchase. Once accepted
by the supplier, it becomes a legal contract for delivery of the goods according to the terms
and conditions specified in the purchase agreement. The purchase order is prepared from the
purchase requisition or the quotations and from any other additional information needed.
A copy is sent to the supplier, copies are retained by purchasing and are also sent to other
departments such as accounting, the originating department, and receiving. Purchase orders
are often submitted electronically to the supplier, and are retained internally as electronic
files that are accessible by all departments, as a substitute for paper purchase orders.
Following up and delivery The purchasing department is responsible for ensuring
that suppliers deliver the items ordered on time. If there is doubt that delivery dates can
be met, purchasing must be notified in time to take corrective action. This might involve
expediting transportation, alternate sources of supply, working with the supplier to solve
its problems, or rescheduling production.
The purchasing department is also responsible for working with the supplier on any
changes in delivery requirements. demand for items changes with time, and it may be
necessary to expedite certain items or push delivery back on some others. The buyer must
keep the supplier informed of the true requirements so that the supplier is able to provide
what is wanted when it is wanted. Outputs from MrP provide messages when items have
been re-prioritized, so that the buyer can determine what action is necessary.
Receiving and accepting goods When the goods are received, the receiving department inspects the goods to be sure the correct items have been sent, are in the right
quantity, and have not been damaged in transit. The receiving department then accepts
the goods. Variances are noted manually on the packing slip, or automatically calculated
using the receiving software. Provided the goods are in order and require no further
inspection, they will be sent to the requisitioning department or to inventory.
If further inspection is required, such as by quality control, the goods are sent to
quality control or held in receiving for inspection. If the goods are received damaged, the
receiving department will advise the purchasing department and hold the goods for further action. Purchasing is notified once the goods have either been inspected or rejected.
Purchasing is then responsible for notifying the supplier and determining the action necessary to return and/or replace the items.
A copy of the receiving report is provided to the purchasing department, noting any
variance or discrepancy from the purchase order. If the receipt completes the purchase
order, it is closed and no longer available for receiving. If the purchase order has not been
received complete, either due to quantities remaining or additional line items, the purchase
order remains open.
Approving supplier’s invoice for payment When the supplier’s invoice is received,
there are three pieces of information that are matched: the purchase order, the receiving
report, and the invoice. The items and the quantities should be the same on all; the prices,
and extensions to prices, should be the same on the purchase order and the invoice. All
discounts and terms of the original purchase order must be checked against the invoice. It
is the job of the purchasing department to work with accounts payable to verify these and to
resolve any differences. Once approved, the invoice is sent to accounts payable for payment.
establIshIng sPecIfIcatIons
determining what exactly to purchase is not necessarily a simple decision. For example,
someone deciding to buy a car should consider how the car will be used, how often, how much
one is willing to pay, and so on. Only then can an individual specify the type of car needed to
make the appropriate purchase. This section looks at the problems that organizations face when
developing specifications of products and the types of specifications that may be used.
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When purchasing an item or a service from a supplier, several factors must be taken
into consideration when specifications are being developed. These can be divided into
three broad categories:
Quantity requirements.
Price requirements.
■■ Functional requirements.
■■
■■
Quantity Requirements
The balance of supply and demand determines the quantity needed. The quantity is important because it will be a factor in the way the product is designed, specified, and manufactured. For example, if the demand was for only one item, it would be designed to be made
at the least cost, or a suitable standard item would be selected. However, if the demand
were for several thousand, the item would be designed to take advantage of economies of
scale, thus satisfying the functional needs at a better price.
Price Requirements
The price specification represents the economic value that the buyer puts on the item,
and the amount the company is willing to pay. If the item is to be sold at a low price,
the manufacturer will not want to pay a high price for a component part. The economic
value placed on the item must relate to the use of the item and its anticipated selling
price.
Functional Requirements
Functional specifications are concerned with the end use of the item and what the
item is expected to do. By their very nature, functional specifications are the most
important of all categories and govern the others. They are also the most difficult to
define. To be successful, they must satisfy the real need or purpose of an item. In many
cases, the real need has both practical and aesthetic elements to it. A coat is meant to
keep one warm, but under what circumstances does it do so and what other functions is
it expected to perform? How cold must it get before one needs a coat? On what occasions will it be worn? Is it for working or dress wear? What color and style should it
be? What emotional needs is it expected to fill? In the same way, the question must
be asked of what practical and aesthetic needs a component of a manufactured item is
expected to satisfy.
Functional specifications and quality Functional specifications are closely tied
to the quality of a product or service. There are many definitions of quality, but they all
center on the idea of user satisfaction. On this basis, it can be said that an item has the
required quality if it satisfies the needs of the user.
There are four phases to providing user satisfaction:
1. Quality and product planning.
2. Quality and product design.
3. Quality and manufacturing.
4. Quality and use.
Product planning is involved with decisions about which products and services a
company is going to market. It must decide the market segment to be served, the product features and quality level expected by that market, the price, and the expected sales
volume. The basic quality level is specified by senior managers according to their understanding of the needs and wants of the marketplace. The success of the product depends
on how well they do this.
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173
The result of the firm’s market studies is a general specification of the product outlining the expected performance, appearance, price, and sales volume of the product. It is
then the job of the product designer to build into the design of the product the quality level
described in the general specification. If this is not properly done, the product may not be
successful in the marketplace.
For manufactured products, it is the responsibility of manufacturing, at a minimum,
to meet the specifications provided by the product designer. If the item is bought, it is
purchasing’s responsibility to make sure the supplier can provide the required quality
level. For purchasing and manufacturing, quality means conforming to specifications or
requirements.
To the final user, quality is related to his or her expectation of how the product should
perform. Customers do not care why a product or service is defective. They expect satisfaction. If the product is what the customer wants, well designed, well made, and well
serviced, the quality is satisfactory.
Functional specifications should define the quality level needed. They should
describe all those characteristics of a product determined by its final use.
Function, quantity, service, and price are interrelated. It is difficult to specify one
without consideration of the others. The final specification is a compromise of all four,
and the successful specification is the best combination. However, functional specifications ultimately are the ones that drive the others. If the product does not perform adequately for the price, it will not sell.
Value analysis Value analysis as defined by APICS Dictionary, 14th edition is “the
systematic use of techniques that identify a required function, establish a value for that
function, and finally provide that function at the lowest overall cost.” Teams of engineers,
users, production personnel, and suppliers analyze parts to challenge current specifications
and identify redundant or unnecessary features. This form of supply chain collaboration
during the design process can be instrumental in reducing the cost and, more importantly,
improving the overall functionality of the part. A good example of value analysis is the
evolution of the milk bottle as it went from a heavy glass bottle to a plastic jug. The result
is a much cheaper package with improvements in sterility, transportation, and breakage.
This will be discussed in further detail in Chapter 14.
functIonal sPecIfIcatIon descrIPtIon
Functional specification can be described in the following ways or by a combination of them:
1. By brand.
2. By specification of physical and chemical characteristics, material and method of
manufacture, and performance.
3. By engineering drawings.
4. By miscellaneous attributes.
Description by Brand
description by brand is most often used in wholesale or retail businesses but can also
be used extensively in manufacturing. This is particularly true under the following
circumstances:
Items are patented, or the process is secret.
The supplier has special expertise that the buyer does not have.
■■ The quantity bought is so small that it is not worth the buyer’s effort to develop
specifications.
■■ The supplier, through advertising or direct sales effort, has created a preference on the
part of the buyer’s customers or staff.
■■
■■
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When buying by brand, the customer is relying on the reputation and integrity of the
supplier. The assumption is that the supplier wishes to maintain the brand’s reputation and
will maintain and guarantee the quality of the product so repeat purchases will give the
buyer the same satisfaction.
Most of the objections to purchasing by brand center on cost. Branded items, as a
group, usually have price levels that are higher than nonbranded items. It may be less
costly to develop specifications for generic products than to rely on brands. The other
major disadvantage to specifying by brand is that it restricts the number of potential
suppliers and reduces competition. Consequently, the usual practice, when specifying
by brand, is to ask for the item by brand name or equivalent. In theory, this allows for
competition.
Description by Specification
There are several ways of describing a product, but whatever method is used, description
by specification depends on the buyer describing in detail exactly what is wanted. One or
more of the following is typically used:
Physical and chemical characteristics. The buyer must define the physical and
chemical properties of the materials wanted. Petroleum products, pharmaceuticals, and
paints are often specified in this way.
■■ Material and method of manufacture. Sometimes the method of manufacture determines the performance and use of a product. For example, hot- and cold-rolled steels
are made differently and have different characteristics.
■■ Performance. This method is used when the buyer is primarily concerned with what
the item is required to do and is prepared to have the supplier decide how performance
is to be attained. For example, a water pump might be specified as having to deliver so
many gallons per minute. Performance specifications are relatively easy to prepare and
take advantage of the supplier’s special knowledge.
■■
Whatever the method of specification, there are several characteristics of description
by specification:
To be useful, specifications must be carefully designed. If they are too loosely
drawn, they may not provide a satisfactory product. If they are too detailed and
elaborate, they are costly to develop, are difficult to inspect, and may discourage
possible suppliers.
■■ Specifications must allow for multiple sources and for competitive bidding.
■■ If performance specifications are used, the buyer is assured that if the product does not
give the desired results, the seller is responsible. They provide a standard for measuring and checking the materials supplied.
■■ not all items lend themselves to specification. For example, it may not be easy to
specify color schemes or the appearance of an item.
■■ An item described by specification may be no more suitable, and a great deal more
expensive, than a supplier’s standard product.
■■ If the specifications are set by the buyer, they may be expensive to develop. They will
be used only when there is sufficient volume of purchases to warrant the cost or where
it is not possible to describe what is wanted in any other way.
■■
Standard specifications Standard specifications have been developed as a result
of much study and effort by governmental and nongovernmental agencies. They usually
apply to raw or semifinished products, component parts, or the composition of material.
In many cases, they have become de facto standards used by consumers and by industry.
When SAE 10W30 motor oil is purchased for a car, a standard grade of motor oil is being
specified as established by the Society of Automotive Engineers. Most of the electrical
products purchased in the united States are manufactured to underwriters laboratory
(ul) standards.
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There are several advantages to using standard specifications. First, they are widely
known and accepted and, because of this, are readily available from most suppliers.
Second, because they are widely accepted, manufactured, and sold, they are lower in price
than nonstandard items. Finally, because they have been developed with input from a broad
range of producers and users, they are usually adaptable to the needs of many purchasers.
Market grades are a type of standard specification usually set by the government and
used for commodities and foodstuffs. Eggs, for example, are purchased by market grade—
small, medium, or large.
Engineering Drawings
Engineering drawings describe in detail the exact configuration of the parts and the
assembly. They also give information on such things as finishes, tolerances, and material to be used. These drawings are a major method of specifying what is wanted and
are widely used because often there is no other way to describe the configuration of
parts or the way they are to fit together. They are produced by the engineering design
department and are expensive to produce, but they give an exact description of the part
required.
Miscellaneous Attributes
There are a variety of other methods of specification, including the famous phrase, “Give
me one just like the last one.” Sometimes samples are used, for example, when colors or
patterns are to be specified. Often a variety of methods can be used, and the buyer must
select the best one.
The method of description is determined by communication with the supplier. How
well it is done will affect the success of the purchase and sometimes the price paid.
selectIng suPPlIers
The objective of purchasing is to get all the right things together: quality, quantity, delivery, and price. Once the decision is made about what to buy, the selection of the right
supplier is the next most important purchasing decision. A good supplier is one that has
the technology to make the product to the required quality, has the capacity to make the
quantities needed, and can run the business well enough to make a profit and still sell a
product competitively.
Sourcing
There are three types of sourcing: sole, multiple, and single.
1. Sole sourcing implies that only one supplier is available because of patents, technical
specifications, raw material, location, and so forth.
2. Multiple sourcing is the use of more than one supplier for an item. The potential
advantages of multiple sourcing are that competition will result in lower price and
better service and that there will be a continuity of supply.
3. Single sourcing is a planned decision by the organization to select one supplier for
an item when several sources are available. It is intended to produce a long-term
partnership. This is discussed at more length in Chapter 15, in the section on supplier
partnerships.
Factors in Selecting Suppliers
The previous section discussed the importance of function, quantity, service, and price
specifications. These are what the supplier is expected to provide and are the basis
for selection and evaluation. Considering this, there are several factors in selecting a
supplier.
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Technical ability does the supplier have the technical ability to make or supply the
product wanted? does the supplier have a program of product development and improvement? Can the supplier assist in improving the products? These questions are important
since, often, the buyer will depend upon the supplier to provide product improvements
that will enhance or reduce the cost of the buyer’s products. Sometimes the supplier can
suggest changes in product specification that will improve the product and reduce cost.
Manufacturing capability Manufacturing must be able to meet the specifications
for the product consistently while producing as few defects as possible. This means that
the supplier’s manufacturing facilities must be able to supply the quality and quantity of
the products wanted. The supplier must have a good quality assurance program, competent and capable manufacturing personnel, and good manufacturing planning and control
systems to ensure timely delivery. These are important in ensuring that the supplier can
supply the quality and quantity wanted.
Reliability In selecting a supplier, it is desirable to pick one that is reputable, stable,
and financially strong. If the relationship is to continue, there must be an atmosphere
of mutual trust and assurance that the supplier is financially strong enough to stay in
business.
After-sales service If the product is of a technical nature or likely to need replacement
parts or technical support, the supplier must have a good after-sales service. This should
include a good service organization and inventory of service parts.
Supplier location Sometimes it is desirable that the supplier be located near the
buyer, or at least maintain an inventory locally. A close location helps shorten delivery
time and means emergency shortages can be delivered quickly.
Lean capabilities Companies competing in a lean environment depend on suppliers
to quickly deliver small quantities of product, to keep inventories at a low level. Today’s
companies operate with very little inventory of raw materials and require accurate, ontime deliveries from their suppliers. Suppliers who simply keep extra inventory to meet
these demands will soon have increased costs and pressures to increase their prices.
Buyers in a lean environment need suppliers who value their new relationship, working
in partnership to remove waste from the system. As a result, these suppliers need to have
information and delivery systems in place that allow them to quickly ship exactly what the
customer needs without increased cost or effort. lean production is discussed further in
Chapter 15.
Other considerations Sometimes other factors such as credit terms, reciprocal business, supplier health and safety record, and willingness of the supplier to hold inventory
for the buyer should be considered.
Price The supplier should be able to provide competitive prices. This does not necessarily mean the lowest price. It is one that considers the ability of the supplier to provide
the necessary goods in the quantity and quality wanted, at the time wanted, as well as any
other services needed.
The total landed cost of an item includes the price paid plus all the handling and
delivery costs associated with getting the product to production. A buyer will often get a
price and per unit transportation discount by ordering in larger quantities. However, the
total cost may increase when the costs of storage and inventory are included.
A low landed cost still may not be a good decision when the total cost of ownership to the company is considered. For example, a carpenter will pay a lower price for a
lower grade of wood. However, the time spent on sorting out knots and defects and the
decreased yield of good material will incur production-related costs, which will increase
the total cost of the final product, perhaps canceling any savings made in price. The total
cost concept looks at the total costs of an item, and not at just the price paid for materials.
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In today’s supply chain environment, the type of relationship between the supplier and the buyer is crucial to both. Ideally, the relationship will be ongoing with a
mutual dependency. The supplier can rely on future business, and the buyer will be
ensured of a supply of quality product, technical support, and product improvement.
Communications between buyer and supplier must be open and complete so both parties understand the problems of the other and can work together to solve problems to
their mutual advantage. Thus, supplier selection and supplier relations are of the utmost
importance.
Identifying Suppliers
A major responsibility of the purchasing department is to continue to research all available
sources of supply. Some aids for identifying sources of supply include:
Salespersons of the supplier company.
Internet.
■■ Catalogues.
■■ Trade magazines.
■■ Trade directories.
■■ Information obtained by the salespeople of the buyer firm.
■■
■■
Final Selection of Supplier
Some factors in evaluating potential suppliers are quantitative, and a dollar value can be
put on them. Price and landed cost are obvious examples. Other factors are qualitative and
require some judgment to determine them. These are usually specified in a descriptive
fashion. The supplier’s technical competence might be an example.
The challenge is finding some method of combining these two major factors that will
enable a buyer to pick the best supplier. One method involves a supplier ranking method,
as follows:
1. Select those factors that must be considered in evaluating potential suppliers.
2. Assign a weight to each factor. This weight determines the importance of the factor in
relation to the other factors. usually a scale of 1 to 10 is used. If one factor is assigned
a weight of 5 and another factor a weight of 10, the second factor is considered twice
as important as the first. When developing the factors and their weights, the buyer
can use input from the people who will be affected by the supplier selection. This will
help the buyer in making a more informed decision and will improve the acceptance
of the new supplier by the users.
3. rate the suppliers for each factor. This rating is not associated with the weight.
rather, suppliers are rated on their ability to meet the requirements of each factor.
Again, usually a scale of 1 to 10 is used.
4. rank the suppliers. For each supplier, the weight of each factor is multiplied by the
supplier rating for that factor. For example, if a factor had a weight of 8 and a supplier
was rated 3 for that factor, the ranking value for that factor would be 24. The supplier
rankings are then added to produce a total ranking. The suppliers can then be listed by
total ranking and the supplier with the highest ranking chosen.
Figure 7.1 shows an example of this method of selecting suppliers. Supplier B has the
highest total of 223; however, supplier d comes in a very close second with 222. The normal practice when using the ranking method is to eliminate the bottom ranking suppliers
from consideration, allowing management to make a simpler decision.
The supplier ranking method is an attempt to quantify those things that are not quantified by nature. It attempts to put figures on subjective judgment. It is not a perfect method,
but it forces the buying company to consider the relative importance of the various factors.
When the method includes the input of many people in determining the relative weights,
agreement on the final selection will be improved.
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Factor
Weight
Suppliers
Function
Cost
Service
Technical
Assistance
Credit
Terms
Rating of
Suppliers
Ranking of
Suppliers
A
B
C
D
A
B
C
D
10
8
8
8
3
9
10
5
4
6
9
5
6
10
7
80
24
72
100
40
32
60
72
40
60
80
56
5
7
9
4
2
35
45
20
10
2
4
3
6
8
8
6
12
16
219
223
204
222
Total (rank of suppliers)
Figure 7.1 Supplier rating.
PrIce determInatIon
As mentioned previously, price is not the only factor in making purchasing decisions, but
can be the determining factor if all other things are equal. In the average manufacturing
company, purchases account for about 50% of the cost of goods sold, and any savings
made in purchase cost has a direct influence on profits. Best price would include the best
mixture of function, quantity, service, and price characteristics.
Basis for Pricing
The term fair price is sometimes used to describe what should be paid for an item. But
what is a fair price? One answer is that it is the lowest price at which the item can be
bought. However, there are other considerations, especially for repeat purchases where
the buyer and seller want to establish a good working relationship. One definition of a fair
price is one that is competitive, gives the seller a profit, and allows the buyer ultimately to
sell at a profit. Sellers who charge too little to cover their costs will not stay in business.
To survive, they may attempt to cut costs by reducing quality and service. In the end, both
the buyer and seller must be satisfied.
Since the objective is to pay a fair price and no more, it is good to develop some basis
for establishing what a fair price is.
Prices have an upper and a lower limit. The market decides the upper limit, as what
buyers are willing to pay is based on their perception of demand, supply, and their needs.
The seller sets the lower limit, and it is determined by the costs of manufacturing and
selling the product and profit expectation. If buyers are to arrive at a fair price, they must
develop an understanding of market demand and supply, competitive prices, and the methods of arriving at a cost.
One widely used method of analyzing costs is to break them down into fixed and
variable costs. Fixed costs are costs incurred no matter the volume of sales. Examples are
equipment depreciation, taxes, insurance, and administrative overhead. Variable costs
are those directly associated with the amount produced or sold. Examples are direct labor,
direct material, and commissions of the sales force.
Total cost = fixed cost + variable cost per unit (number of units2
total cost
unit cost =
number of units
fixed cost
=
+ variable cost per unit
number of units
The preceding formula shows that as the number of units produced increases, the
unit cost decreases. This is an important factor when determining price. Buyers can lower
the unit price paid by increasing the volume per order, using longer-term contracts or
Purchasing
179
DOLLARS
Revenue
Total Cost
Loss Profit
Fixed Cost
Break-Even Point
SALES VOLUME
Figure 7.2 Break-even analysis.
through the standardization of parts. Sellers will offer quantity discounts to encourage
larger orders, also taking advantage of this reduction in unit cost. Quantity discounts are
discussed later in Chapter 10, and standardization is discussed in Chapter 14.
Figure 7.2 shows the relationship of fixed and variable costs to sales volume and
how revenue will behave. The sum of the fixed and variable costs is labeled Total Cost
on the graph. The third line represents the sales revenue. Where this line intercepts the
total cost line, revenue equals total cost, and profit is zero. This is called the break-even
point. When the volume is less than the break-even point, a loss is incurred; when the
volume is greater, a profit is realized. The break-even point occurs where the revenue
equals the total cost.
revenue = Total cost
1Price per unit2 1number of units sold2 = fixed cost + 1variable cost per unit2
* 1number of units2
example Problem
To make a particular component requires an overhead (fixed) cost of $5000 and a
variable unit cost of $6.50 per unit. What is the total cost and the average cost of
producing a lot of 1000? If the selling price is $15 per unit, what is the break-even
point?
Answer
Total cost = $5000 + 1$6.50 * $10002 = $11,500
Average cost = $11,500 , 1000 = $11.50 per unit
Break-even point: Let X = number of units sold
$15X = $5000 + $6.5X
$8.50X = $5000
Break-even occurs when 588.2 units are made and sold.
Competitive Bidding
Competitive bidding occurs when a buyer compares the price of a product from various
suppliers and simply chooses the lowest price. This can be the formal process of sending out quotations and analyzing the results or simply comparing catalogue or advertised
prices. The process does take some time, and a number of sources must be available. At
least three sources are desired to make a good comparison. Competitive bidding also
requires that the product be well specified and widely available. Items such as nuts and
bolts, gasoline, bread, and milk are usually sourced using competitive bidding.
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healthcare PrIce negotIatIons
As in other industries, price is dependent on various factors. However, in healthcare, one factor that needs to be
realized is the amount the institution will reimburse for
that particular product. The cost to provide patient care
includes the cost of goods and the cost to administer care.
While most contracts within healthcare are negotiated
toward a fixed cost, organizations need to make certain
they cover those costs and have an understanding of what
reimbursement they will receive from providers.
The second piece is the ability to benchmark pricing.
Some institutions have access to pricing data that is available from various sources. By using this data, it gives the
institution the ability to see what the upper and lower limits
are on particular products. This data can be used to begin
price negotiation to ensure you are covering all the costs.
The final portion is the availability of the product.
Organizations need to partner with vendors/suppliers who
sufficiently supply to meet their needs and deliver it in a
timely, cost-effective manner.
Quality products at the right price and delivered at
the right time.
Submitted by:
Luis A. Richard
AVP, Supply Chain Management Health-Quest Systems, Inc.
Poughkeepsie, New York
Price Negotiation
Prices can be negotiated if the buyer has the knowledge and the clout to do so. A small
retailer probably has little of the latter, but a large buyer may have much. Through negotiation, the buyer and seller try to resolve conditions of purchase to the mutual benefit of
both parties. Skill and careful planning are required for the negotiation to be successful. It
also takes a great deal of time and effort, so the potential profit must justify the expense.
One important factor in the approach to negotiation is the type of product. There are
four categories:
1. Commodities. Commodities are materials such as copper, coal, wheat, meat, and
metals. Price is set by market supply and demand and can fluctuate widely. negotiation
is concerned with contracts for future prices.
2. Standard products. These items are provided by many suppliers. Since the items
are standard and the choice of suppliers large, prices are determined on the basis of
published prices. There is not much room for negotiation except for large purchases.
3. Items of small value. These are items such as maintenance or cleaning supplies and
represent purchases of such small value that price negotiation is of little purpose. The
prime objective should be to keep the cost of ordering low. Firms will negotiate a
contract with a supplier that can supply many items and set up a simple ordering system that reduces the cost of ordering.
4. Made-to-order items. This category includes items made to specification or
for which quotations from several sources are received. These can generally be
negotiated.
ImPact of materIal requIrements
PlannIng on PurchasIng
The text in this chapter has described the traditional role and responsibilities of purchasing. This section will study the effect material requirements planning has on the purchasing function and the changing role of purchasing.
Purchasing can be separated into two types of activities: (1) procurement and
(2) supplier scheduling and follow-up. Much of what has been covered in this chapter is
in the area of procurement. Procurement includes the functions of establishing specifications, selecting suppliers, price determination, and negotiation. Supplier scheduling and
follow-up are concerned with the release of orders to suppliers, working with suppliers
to schedule delivery, and follow-up. The goals of supplier scheduling are the same as
those of production activity control: to execute the master production schedule (MPS) and
the material requirements plan (MrP), ensure good use of resources, minimize work-inprocess inventory, and maintain the desired level of customer service.
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181
Planner/buyer concept In a traditional system, the material requirements planner
releases an order either to production activity control or to purchasing. Purchasing issues
purchase orders based on the material requirements plan. Production activity control prepares shop orders, schedules components into the work flow, and controls material progress
through the plant. When plans change, as they invariably do, the production planner must
advise the buyer of the change, and the buyer must advise the supplier. The production
planner is in closer, more continuous contact with material requirements planning and frequently changing schedules than is the buyer. To improve the effectiveness of the planner/
buyer activity, many companies have combined the two functions of buying and planning
into a single job done by one person. Planner/buyers do the material planning for the
items under their control, communicate the schedules to their suppliers, follow up, resolve
problems, and work with other planners and the master scheduler when delivery problems
arise. The planner/buyer is responsible for the following:
determining material requirements.
developing schedules.
■■ Issuing shop orders.
■■ Issuing material releases to suppliers.
■■ Establishing delivery priorities.
■■ Controlling orders in the factory and to suppliers.
■■ Handling all the activities associated with the buying and production scheduling functions.
■■ Maintaining close contact with supplier personnel.
■■
■■
Because the roles of production planning and buying are combined, there is a smoother
flow of information and material between the supplier and the factory. The planner/buyer
has a keener knowledge of factory needs than the buyer does and can better coordinate the
material flow with suppliers. At the same time, the planner/buyer is better able to match
material requirements with the supplier’s manufacturing capabilities and constraints.
Contract buying usually an MrP system generates frequent orders for relatively small
quantities. This is particularly true for components that are ordered lot-for-lot. It can be costly,
inefficient, and sometimes impossible to issue a new purchase order for every weekly requirement. The alternative is to develop a long-term contract with a supplier and to authorize
releases against the contract. Often suppliers are given a copy of the material requirements plan
so they are aware of future demands. The buyer then issues a release against the schedule. This
approach is efficient and cost-effective but requires close coordination and communication
with the supplier. Again, contract buying can be managed very well by a planner/buyer.
Supplier responsiveness and reliability Because material requirements often change,
suppliers must be able to react quickly to change. They must be highly flexible and reliable so
they can react quickly to changes in schedules. responsiveness and reliability are qualitative
factors that must be taken into consideration when selecting suppliers. long-term contracts
ensure suppliers a given amount of business and commits them to allocating that amount of
their capacity to the customer. Suppliers are more responsive to customer needs and can react
quickly to changes in schedules. Because customers know the capacity will be available when
needed, they can delay ordering until they are more sure of their requirements.
Close relationship with suppliers Contract buying and the need for supplier flexibility and reliability mean the buyer/supplier relationship must be close and cooperative.
There must be excellent two-way communication, cooperation, and teamwork. Both parties have to understand their own and the other’s operations and problems.
The planner/buyer and the supplier counterpart, often the supplier’s production planner, must work on a daily or weekly basis to ensure both parties are aware of any changes
in material requirements or material availability.
Electronic data interchange Electronic data interchange (EDI) enables customers
and suppliers to electronically exchange transaction information such as purchase orders,
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healthcare suPPly and dIstrIbutIon
Healthcare is no different to the supply and demand
process. It is all based on the need and current volume an
institution has when taking care of patients. By working
with suppliers and distributors, healthcare institutions can
begin to forecast their needs based on trends or ordering
patterns. The supplier/distributor will then provide the
institution the inventory by following the defined replenishment process. That process typically follows:
■■ Purchase request
■■ Purchase order
■■ Supplier/distributor delivering product
■■ Receipt of product
■■ Product replenishment to either a warehouse, store
room, or par location
■■ Par/cycle count
This process can either happen manually or electronically
depending on the organizations preference. Organizations
will keep a certain amount of inventory in par locations and
replenish from either a store room or warehouse.
Submitted by:
Luis A. Richard
AVP, Supply Chain Management Health-Quest Systems, Inc.
Poughkeepsie, New York
invoices, and material requirements planning information. This eliminates time-consuming
paperwork and facilitates easy communication between planner/buyers and suppliers.
Vendor-managed inventory In recent years there has been an increase in the purchasing approach known as vendor-managed inventory (VMI). In this concept, a supplier maintains an inventory of certain items in the customer’s facility. The supplier owns the inventory
until the customer actually withdraws it for use, after which the customer pays for the amount
consumed. The customer does not have to order any of the inventory, as the supplier is
responsible for maintaining an adequate supply in the facility for customer use. This approach
is most commonly used for lower-value products that have a relatively standard design, such
as fasteners, standard electrical equipment, and so forth, although it can also be used for
large-value items as well. Vendor-managed inventory reduces the need for many small MrP
order releases, and increases the collaboration of the supply chain.
Internet Internet technology has changed the purchasing function in many ways. There
are three variations of networks used: internet, intranet, and extranet. The internet is most
commonly used and is open to the general public. An intranet is an internal network
that is normally used within the boundaries of a company. It may stretch across many
manufacturing sites or even countries. Much of the data shared in this environment is considered sensitive, and therefore, access is usually limited to people within the company.
Extranet is an intranet shared by two or more companies. Each participating company
moves certain data outside of a private intranet to the extranet, making it available only
to the companies sharing the extranet. For example, a supplier may be provided with
information such as the planned order releases from ErP or the stock status of an item.
Integration of two distinct ErP systems via the internet, to enable real-time communication between a customer and a supplier, is also now possible.
envIronmentally resPonsIble PurchasIng
The fifth category of purchasing objectives introduced at the beginning of this chapter is
the objective for purchasing departments to minimize the impact their organization has on
the environment, that is, environmentally responsible purchasing. Purchasing is responsible for managing waste products in most organizations since they have:
First-hand knowledge of price trends for waste products.
Contact with salespeople who are an excellent source of information as to possible
uses of waste material.
■■ Familiarity with the company’s own needs, or uses for materials within the
organization.
■■ Knowledge of legislation involving the transportation and handling of environmentally
sensitive materials.
■■
■■
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183
Reduce, Reuse, and Recycle is a widely used phrase to guide people in lowering the
impact they have on the environment. Properly understood and implemented, the 3 rs can
also reduce expenses and increase profits.
Reduce
reducing the use or generation of materials, whether hazardous or scrap, is the most
environmentally friendly of the 3 rs. The purchasing department, having direct contact
with suppliers, is the first to learn of new environmentally friendly materials. lead-free
solder and water-based solvents are just two examples of materials that have been developed by suppliers to help their customers reduce their environmental impact and reduce
their costs. lean principles (discussed further in Chapter 15), when applied to suppliers,
involves reducing waste for all stages of the supply chain. The use of returnable racks
or packaging is widely used in many industries, reducing both costs and environmental
impact.
Suppliers should be consulted when developing material specifications to reduce
costs by cutting waste or recommending bulk storage and handling systems.
Reuse
The next most effective step is to reuse materials wherever possible. Scrap from one
process may be reused directly within the organization or can be slightly processed for
reuse in another process. Corrugated cartons can be slit and crushed for use as packing
material in the shipping department. Many manufactured products are cut from a continuous sheet resulting in waste material. However, smaller products can often be nested
between the cut-outs to make other, smaller products, reducing the need for raw materials and also the amount of material to be sent for disposal. Another category of reuse is
by-products, which are saleable products made from what was previously considered
waste. It was not too long ago that butchers had to find a use for a nonsaleable product,
chicken wings!
An example from the food packaging industry is the shipping of glass jars in boxes
preprinted with finished product artwork. The empty jars are removed from their containers as they enter the cleaning and filling station. The empty boxes are sent to the end of
the filling process, where the finished product is placed in the boxes. A final description
and batch code are then printed on each box. The company never has to deal with the handling and disposal of the boxes used to ship the empty jars. The boxes have two uses: the
shipment from glass manufacturer to the food processor and from the food processor as
finished product.
Recycle
This is the most public and least effective of the 3 rs, yet purchasing can maximize the
benefits of recycling through good management. Suppliers are often the best source of
information on the disposal of scrap materials and often will buy materials back for reprocessing. This does require good management to keep materials in their most useful form.
Some liquids, notably chlorinated solvents, are difficult to dispose of. They should be
kept separate from other valuable liquids, such as used machine oils. Contamination of a
waste stream can turn the resale of a valuable waste product into an expense and should
be avoided at all costs. All materials destined for recycling should be kept separate from
other materials where possible.
exPansIon of PurchasIng Into suPPly
chaIn management
As computers and software (ErP, for example) have become more powerful and effective, information flows have become easier and the ability to handle large amounts of data
has become more feasible. This condition has allowed companies to expand their planning
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and control perspectives to include upstream (suppliers) and downstream (distributors and
customers) entities. This concept of the supply chain has four major components that are
managed:
The flow of physical materials from suppliers, downstream through the company
itself, and finally to distributors and/or customers.
■■ The flow of money upstream from customers back to the companies and suppliers.
■■ The flow of information up and down through the stream.
■■ The flow of products back (upstream) from the customers, typically for repairs or
recycling. This is known as reverse logistics.
■■
Other entities also have an impact on the supply chain for a company. A major
example is governments, which can impact the supply chain both positively and negatively. The word governments is plural since many modern supply chains are global in
nature, often including companies and customers from many different countries around
the world.
Although several impacts on the supply chain perspective have been observed and
formalized, two in particular have become formalized and noteworthy:
Customer relationship management (CRM) includes several activities with the
intent to build and maintain a strong customer base. Customer wants and needs are
assessed and cross-functional teams from the company work to align company activities around those customer needs.
■■ Supplier relationship management (SRM) is similar to CrM, with the focus for
these activities being the building and maintaining of close, long-term relationships
with key suppliers.
■■
One critical reason for developing formal links and relationships in the supply chain
is to help control the bullwhip effect. This effect occurs when there is uncertainty in the
supply chain based on the use of forecasts, and that uncertainty is then exaggerated as
material moves through the supply chain.
The effect can produce large fluctuations in demand for raw materials based on
relatively small changes in demand from the customer end of the supply chain. This is
illustrated in Figure 7.3. A small fluctuation at the customer causes a ripple effect as the
change passes through each node of the supply chain, exaggerated by lead times and differences in lot sizes. Managing the supply chain with visibility of data (information flow)
and building flexibility and agility across the supply chain, can substantially reduce large
fluctuations.
The management required to effectively manage the supply chain is heavily based on
managing data and inventory, but there are other aspects as well. Good strategic planning,
focusing on anticipating and preparing for disruptions, and sharing those risks among
other entities in the supply chain represent examples of the types of supply chain management that have been developed.
Another key issue that has grown in importance with the growth of supply chain is the
focus on ethics. People in purchasing and supply chain positions often have responsibility
for managing the flow of a great deal of money, and as a result it becomes quite important to have honest and ethical conduct on the part of those people. Many companies, and
countries, have developed strict codes and rules of conduct to ensure consistent and ethical treatment of the supply chain activities and reported measures.
Supplier
Factory
Distributor
Figure 7.3 Bullwhip effect.
Retailer
Customer
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185
some organIzatIonal ImPlIcatIons of
suPPly chaIn management
Organizations that move their perspective away from traditional purchasing toward supply
chain management must recognize that their perspective toward managing the entire organization must also change. For instance, most organizations that have adopted a supply
chain perspective find the following:
Their cost focus has altered dramatically. Often decisions are not based on just product
price but instead on total cost and value. This implies an integrated view of price, quality,
serviceability, durability, and any other characteristic that the company places on total
value. It also can include transportation, storage, and material handling costs. To accomplish this changed perspective, organizations have adopted techniques of process analysis,
value stream analysis, and mutual value analysis between the company and its suppliers.
■■ Cross-functional teams are now used to plan and control the supply chain. These teams
are made up of representatives from various departments, including production, quality
control, engineering, finance, and purchasing. Cross-functional teams make decisions
more quickly than traditional departmental organizations and also are likely to consider
overall benefits to the company, not just benefits to the individual departments.
■■ The decision on whether the purchasing function should be centralized or decentralized can be impacted by supply chain management. Centralized purchasing allows
organizations to be able to take advantage of volume discounts and maintain centralized control over suppliers, pricing, and so forth. However, the supply chain partnership concept often leads companies to consider a more decentralized approach, which
can facilitate more of a relationship between purchasing and local suppliers. Integrated
systems such as ErP can offer the benefits of centralized purchasing, while allowing
purchasing to be located in multiple facilities. Both approaches have advantages, and
the decision should be made based on the strategic plan of the company.
■■ decision making has changed from the “I say and you do” or a negotiated perspective
with suppliers, to one of “let’s talk about the best way to handle this and make a mutually advantageous decision.” This also implies supplier contracts extending into the
long-term future.
■■ Information sharing has changed from simply giving out information about the order
to the sharing of important information about the business itself, which requires
mutual trust and cooperation between entities.
■■ Measurement systems look at all aspects of the supply chain and not just supplier
performance.
■■ There is a growth in electronic business (e-business). This implies using the internet
more for handling business information flows and transactions.
■■ The environment must be considered in the acquisition, storage, use, and disposal of
all materials
■■
Savings Can Be Substantial
There are many advantages associated with an effective supply chain perspective. Some of
these savings include the following:
More effective product specification, allowing for efficient product substitutions and
product specifications focused on fitness of use.
■■ Better leveraging of volume discounts and supplier consolidation.
■■ long-term contracts with efficient communication systems, significantly reducing the
administrative cost of ordering and order tracking.
■■ More effective use of techniques such as electronic commerce, using credit cards for
payments, and blanket ordering.
■■ reducing environmental costs by avoiding potentially hazardous materials and exercising the 3 rs of reduce, reuse, and recycle.
■■
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summary
Purchasing has always been an important function in any company, especially manufacturers who use a lot of raw materials or materials that are difficult to obtain. Purchasing
needs to continue to get the right products at the right time and at the best price but,
the function is changing. The steps in the purchasing cycle are still necessary but many
manual activities such as writing POs, getting information on products, and communicating with suppliers have been sped up through use of the internet and computerization. The
reduction in routine clerical activity allows time to take a more strategic view of the organization and have an increasing impact on profits. Viewing the supply chain as an integrated function, outsourcing, and lean production are three management influences that
have encouraged purchasing to improve their relations with suppliers and to take a more
active role in the scheduling and flow of products. Purchasing also has the opportunity to
take a lead role in reducing the environmental impact of a company by working with suppliers and the use of environmentally friendly materials.
Key terms
Break-even point 179
Bullwhip effect 184
By-product 183
Commodities 180
Contract buying 181
Customer relationship management
(CrM) 184
Electronic data interchange (EdI) 181
Engineering drawings 175
Enterprise resource planning
(ErP) 170
Environmentally responsible
purchasing 182
Extranet 182
Fair price 178
Fixed costs 178
Functional specifications 172
Internet 182
Intranet 182
landed cost 176
Maintenance, repair, and operating
supplies (MrO) 170
Multiple sourcing 175
Offshoring 169
Outsourcing 169
Planner/buyers 181
Purchasing cycle 170
Purchasing requisition 170
reduce, reuse, and recycle 183
request for quote (rFQ) 170
reverse logistics 184
Single sourcing 175
Sole sourcing 175
Standard specifications 174
Supplier flexibility and
reliability 181
Supplier ranking 177
Supplier relationship management
(SrM) 184
Total cost of ownership 176
Value analysis 173
Variable costs 178
Vendor-managed inventory
(VMI) 182
questIons
1. What are the five objectives of purchasing?
2. list the seven steps in the purchasing cycle.
3. describe the purposes, similarities, and differences among purchase requisitions, purchase
orders, and requests for quotation.
4. What are the responsibilities of the purchasing department in follow-up?
5. describe the duties of the receiving department upon receipt of goods.
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187
6. Besides functional specifications, what other specifications must be determined? Why is each
important?
7. name two sources of specifications.
8. What is the difference between sole sourcing and single sourcing?
9. describe the advantages and disadvantages of the following ways of describing functional
requirements. Give examples of when each is used.
a. By brand.
b. By specification of physical and chemical characteristics, material and method of manufacture, and performance.
10. What are the advantages of using standard specifications?
11. Why is it important to select the right supplier and to maintain a relationship with that supplier?
12. name and describe the three types of sourcing.
13. describe the factors that should be used in selecting a supplier.
14. What is the bullwhip concept?
15. Type of product is a factor that influences the approach to negotiation. name the four categories of products and state what room there is for negotiation.
16. What are five savings that can result from adopting a supply chain management approach?
17. A company would like to reduce the amount of lead time in some of their soldered electronics.
How can the purchasing department contribute to this endeavor?
18. describe which of the 3 rs has the most beneficial impact on the environment.
Problems
7.1. If purchases were 45% of sales and other expenses were 45% of sales, what would be
the increase in profit if, through better purchasing, the cost of purchases was reduced
to 43% of sales?
7.2. If suppliers were to be rated on the following basis, what would be the ranking of the
two suppliers listed?
Factor
Function
Cost
Technical Assistance
Credit Terms
Weight
7
5
4
1
Rating of Suppliers
Ranking of Suppliers
Supplier A Supplier B Supplier A Supplier B
6
9
9
6
5
7
7
4
7.3. A company is negotiating with a potential supplier for the purchase of 100,000 widgets. The company estimates that the supplier’s variable costs are $5 per unit and
that the fixed costs, depreciation, overhead, and so on, are $50,000. The supplier
quotes a price of $10 per unit. Calculate the estimated average cost per unit. do you
think $10 is too much to pay? Could the purchasing department negotiate a better
price? How?
case study 7.1
Let’s Party!
“let’s party!” is still echoing in your head as you leave your Principles of Purchasing
class. Again you ask yourself, “Why did I ever let myself run for class president?” Most
of the people in the class were good, level-headed individuals who enjoyed a good time
and you enjoyed working with them. But a small group from your class, who were known
on campus as The rowdies, often bullied their way on decisions affecting class activities.
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Chapter seven
The decision to have a year-end party was right up their alley, and class had ended with a
chanting session of “let’s party.” It sounded like a wrestling match to you. Fortunately,
your professor had left the room early to let you discuss with the class the idea of some
kind of year-end get-together.
The rowdies had immediately suggested the Goat’s Ear, a local hangout with not
much to offer but cheap drinks. The rest of your classmates had put forth some other suggestions, but no consensus on a location could be reached between the members of your
executive committee or the rest of the class. If you went to the Goat’s Ear, most of the
sane people in your class wouldn’t attend, and even when you suggested more conventional locations, people couldn’t agree because of factors such as the type of music played.
Since there were only two weeks left until the end of regular classes, you felt that you
had to make arrangements in a hurry. It wasn’t difficult to identify the most popular possible locations, but getting agreement from this group was going to be difficult.
One of your recent lectures was on supplier selection, and your professor had demonstrated the technique called the ranking or weighted-point method. It seemed simple
enough in the lecture, and you had almost embarrassed yourself by asking the question
“Why not just pick the least expensive supplier?” The thought occurred to you that there
just might be some solution to your current problem in the professor’s response, “One of
the hardest things to do in any group, whether a business or a social club, is to get consensus on even the simplest choices.”
assignment
For this exercise, put yourself in the position of the class president described in this case
and complete one of the two following exercises:
exercise 1
1. Perform a supplier rating analysis for the situation. Include at least 10 factors and 4
possible locations.
2. Make the selection as indicated by the analysis.
3. discuss why the analysis led to your selection in step 2 and whether you would
change any of the criteria or weights.
exercise 2
1. Prepare a presentation to be used in class to make a selection for a year-end
get-together.
2. lead a discussion to determine at least four possible locations and 10 factors.
3. Have the class agree on weighting factors for each criteria.
4. Perform the calculations and make the selection.
5. discuss with the class why the analysis led to the selection in step 4 and whether you
would change any of the criteria or weights.
6. Ask the class members whether they feel more in agreement with the decision after
going through this process.
case study 7.2
The Connery Company
When Juan Hernandez was first given the position of head buyer for the Connery
Manufacturing Company, he visualized the job as merely an expansion of his old position
as a commodity buyer. He had no formal training when he took the position, having been
promoted to commodity buyer from his position as inventory clerk, which he had gotten
directly out of high school.
The lack of formal training was not a problem when he first took the job. The Connery
Company was small but growing, and the major concern of the purchasing department
was to obtain adequate purchased material to support the production and the growth in
sales. There was little done in the way of price negotiation. The reason was that there was
Purchasing
189
little competition for their products and all costs could easily be passed along in the product price, leaving room for the healthy profits that have helped Connery grow so rapidly.
As is often the case in these type of situations, the luxury of little competition and
flexibility in pricing was fairly short-lived. The success of the products Connery produced
attracted a lot of attention, and soon Connery found itself in a market with several strong
competitors.
While they still had the advantage of some recognition in the market (“first mover”
advantage, meaning that the first entrant into the market usually has a competitive advantage), that advantageous position was in grave danger of erosion. They also had an advantage in being further down the learning curve, and the quality of their product had always
been quite good. The problem now was cost. Competition was driving down the prices
and maintaining their delivery record at a lower price was rapidly becoming an important
factor in stemming the tide of market share erosion.
These factors were one of the key reasons that Juan was promoted. He was recognized as the best and most experienced of all the buyers, and Mr. Connery recognized the
need to move the procurement activity from one of passive buying into an active and aggressive supply management group.
As a buyer, Juan’s primary responsibility was to get the material they needed, when they
needed it. He primarily was responsible for buying standard components and materials, so he
had hundreds of catalogs from all possible suppliers of these standard commodities. When he
needed to place an order, he would typically use the catalog price or the quoted price from
a supplier as long as they could meet the delivery time he needed. He had little concern for
transportation cost or even quality, since for these standard components the quality from all
possible suppliers was roughly equivalent. There were a few cases in the past when quality
did prove to be a problem, but the supplier could usually respond quickly with an appropriate
replacement. Even though the supplier would typically give Connery credit for any rejected
parts, changing schedules around the problem or carrying safety stock to protect against problems would both end up costing Connery more money.
Soon after the promotion to head buyer, Juan realized the job would be much more
than merely an expansion of his old position. Mr. Connery told Juan he created the position
of head buyer to move the company into a more cost-competitive condition. He wanted
Juan to develop and implement a plan that would attempt to accomplish the following:
reduce purchased raw material inventory levels.
Improve delivery speed and reliability of purchased material.
■■ Improve the quality performance of suppliers.
■■ reduce the overall cost of purchased materials.
■■
■■
These actions were considered to be important if they were to reduce the overall cost
and stay ahead of the pack on price competitiveness.
Juan now realized both the extent and the seriousness of the new position and his responsibility. The following give a little indication of the current position of the company:
Annual cost of goods sold
direct material cost
Inventory (on balance sheet)
Supplied parts transportation expense
number of suppliers
Inventory holding cost
Average total processing cost for products
number of different designs for end product
$14,827,527
$8,517,323
$2,352,117
$256,103
2872
21% per year
3 hours, 27 minutes
72
assignment
1. What additional information should Juan gather to help him develop his plan?
Explain how you would use the information.
2. Once you know the information, develop a plan for Juan.
Chapter
eight
Forecasting and demand
management
introduction
Forecasting is a prelude to planning. Before making plans, an estimate must be made of
what conditions will exist over some future period. How estimates are made, and with
what accuracy, is another matter, but little can be done without some form of estimation.
Why forecast? There are many circumstances and reasons, but forecasting is inevitable in developing plans to satisfy future demand. Most firms cannot wait until orders
are actually received before they start to plan what to produce. Customers usually demand
delivery in reasonable time, and manufacturers must anticipate future demand for products
or services and plan to provide the capacity and resources to meet that demand. Firms that
make standard products need to have saleable goods immediately available or at least to
have materials and subassemblies available to shorten the delivery time. Firms that make
to order cannot begin making a product before a customer places an order but must have
the resources of labor and equipment available to meet demand.
Many factors influence the demand for a firm’s products and services. Although it
is not possible to identify all of them, or their effect on demand, it is helpful to consider
some major factors:
General business and economic conditions.
Competitive factors.
■■ Market trends such as changing demand.
■■ The firm’s own plans for advertising, promotion, pricing, and product changes.
■■
■■
demand management
The primary purpose of an organization is to serve the customer. By effectively serving
customers, the company will often be successful, leading to what financial people identify
as a primary purpose of maximizing company shareholder value. Marketing focuses on
meeting customer needs, but operations, through materials management, must provide the
resources. The coordination of plans by these two parties is demand management.
Demand management is the function of recognizing and managing all demands for
products and/or services. It occurs in the short, medium, and long term. In the long term,
demand projections are needed for strategic business planning of such things as facilities.
In the medium term, the purpose of demand management is to project aggregate demand
for production planning. In the short term, demand management is needed for items and is
associated with master production scheduling. This text is most concerned with the latter.
If material and capacity resources are to be planned effectively, all sources of demand
must be identified. These include domestic and foreign customers, other plants in the same
corporation, branch warehouses, service parts and requirements, promotions, distribution
inventory, and consigned inventory in customers’ locations.
Demand management includes several major activities, all of which are primarily
market driven:
■■
190
Identifying all product and service demand in the defined markets. This includes
forecasting but also involves possible segmenting of markets, classifying customers, and identifying demand that does not add value and therefore should be ignored.
Forecasting and Demand Management
191
PRODUCTION
PLANNING
MARKETPLACE
DEMAND
MANAGEMENT
MASTER
PRODUCTION
SCHEDULE
Figure 8.1 Demand management and the manufacturing planning and control system.
This includes identifying customer desires for existing or possible new product, or
service design and features.
Identifying and understanding all aspects of the market that will potentially impact
customer demand. This includes economic conditions and indicators, governmental
laws and regulations, and sources of existing or potential competition, including possible new competitors.
■■ Synchronizing identified market demand with company capabilities.
■■ Setting priorities for demand when supply will not cover all demand.
■■ Making delivery promises. The concept of available-to-promise was discussed in
Chapter 3.
■■ Interfacing between manufacturing planning and control and the marketplace. Figure 8.1
shows this relationship using a block diagram.
■■ Order processing.
■■
In each of these cases, production (supply) is being planned to react to anticipated
demand as shown by the forecast.
There are several other activities or components to demand management. They
include the following:
Setting and maintaining appropriate customer service levels.
Planning for new product introductions and phase-out of obsolete inventory.
■■ Planning and managing interplant shipments and distribution requirements planning
(discussed in Chapter 11).
■■ Establishing inventory target levels and maintaining them.
■■ Establishing performance metrics for demand and using them to evaluate performance.
■■
■■
A proactive approach to demand management (market driven) includes four major activities, as discussed in Bricks Matter: The Role of Supply Chains in Building Market-Driven
Differentiation by Cecere and Chase:
Sensing the demand.
Shaping demand—based on strategic and market plans.
■■ Shifting demand—when appropriate, sales, marketing, and operations use communication, advertising, pricing, and promotion to shift demand in desirable patterns for
the company. This implies encouraging demand for products or services that may be
easiest to make or are the most profitable, while discouraging demand for products or
services that may be difficult to make or provide little or no profit.
■■ Responding to demand.
■■
■■
Collaborative planning, forecasting, and replenishment As the concepts of
supply chain continue to develop and mature, another approach to identify demand has
been developed. Called Collaborative Planning, Forecasting, and Replenishment
(CPFR), the approach establishes a relationship between trading partners in a supply
chain. They then create a joint business plan from which sales forecasts can be developed
and communicated between the supply chain partners. This approach can be beneficial
in obtaining more accurate demand information within the defined supply chain. This
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Chapter eight
approach tends to be a closed-loop approach, where results are analyzed after plans are
executed. The results from the analysis may then be used to evaluate and possibly improve
the collaborative relationship.
Order processing Order processing occurs when a customer’s order is received.
The product may be delivered from finished goods inventory or it may be made or
assembled to order. If goods are sold from inventory, a sales order is produced authorizing the goods to be shipped from inventory. If the product is made or assembled to
order, the sales department must write up a sales order specifying the product. This may
be relatively simple if the product is assembled from standard components but can be a
lengthy, complex process if the product requires extensive engineering. A copy of the
sales order stating the terms and conditions of acceptance of the order is sent to the customer. Another copy, sent to the master planner, is authorization to go ahead and plan
for manufacture. The master planner must know what to produce, how much to produce,
and when to deliver. The sales order must be written in language that makes this information clear.
demand Forecasting
Forecasts differ depending on what is to be done. They must be made for the strategic
plan, the strategic business plan, the sales and operation plan, and the master production
schedule. As discussed in Chapter 2, the purpose, planning horizons, and level of detail
vary for each.
The strategic plan and the business plan are concerned with overall markets and
the direction of the economy over the next 2 to 10 years or more. Their purpose is to
provide time to plan for those things that take long to change. For production, the strategic plan and the business plan should provide sufficient time for resource planning:
plant expansion, capital equipment purchase, and anything requiring a long lead time
to purchase. The level of detail is not high, and usually forecasts are in sales units,
sales dollars, or capacity. Forecasts and planning will probably be reviewed quarterly
or yearly.
Sales and operations planning is concerned with manufacturing activity for the next
one to three years. For manufacturing, it means forecasting those items needed for production planning, such as budgets, labor planning, long lead time procurement items, and
overall inventory levels. Forecasts are made for groups or families of products rather than
specific end items. Forecasts and plans will probably be reviewed monthly.
Master production scheduling is concerned with production activity from the present to a few months ahead. Forecasts are made for individual items, as found on a master
production schedule, individual item inventory levels, raw materials and component parts,
labor planning, and so forth. Forecasts and plans will probably be reviewed weekly.
characteristics oF demand
In this chapter, the term demand is used rather than sales. The difference is that sales
implies what is actually sold, whereas demand shows market or customer requests.
Sometimes demand cannot be satisfied, and sales will be less than demand.
Before discussing forecasting principles and techniques, it is best to look at some
characteristics of demand that influence the forecast and the particular techniques used.
Demand Patterns
If historical data for demand are plotted against a time scale, they will show any shapes or
consistent patterns that exist. A pattern is the general shape of a time series. Although some
individual data points will not fall exactly on the pattern, they tend to cluster around it.
Figure 8.2 shows a hypothetical historical demand pattern. The pattern shows that
actual demand varies from period to period. There are four reasons for this: trend, seasonality, random variation, and cycle.
Forecasting and Demand Management
193
5
Trend
DEMAND
4
3
2
Seasonal
Demand
1
0
1
2
3
4
1
5
6
7
QUARTERS
2
YEARS
8
9
10
11
12
3
Figure 8.2 Demand over time.
Trend Figure 8.2 shows that demand is increasing in a steady pattern of demand from
year to year. This graph illustrates a linear trend, but there are different shapes, such as
geometric or exponential. The trend can be level, having no change from period to period,
or it can rise or fall.
Seasonality The demand pattern in Figure 8.2 shows each year’s demand fluctuating
depending on the time of year. This fluctuation may be the result of the weather, holiday seasons, or particular events that take place on a seasonal basis. Seasonality is usually thought
of as occurring on a yearly basis, but it can also occur on a weekly or even daily basis. A
restaurant’s demand varies with the hour of the day, and supermarket sales vary with the day
of the week.
Random variation Random variation occurs where many factors affect demand
during specific periods and occur on a random basis. The variation may be small, with
actual demand falling close to the pattern, or it may be large, with the points widely scattered. The pattern of variation can usually be measured, and this will be discussed in the
section on tracking the forecast.
Cycle Over a span of several years and even decades, wavelike increases and decreases
in the economy influence demand. However, forecasting of cycles is a job for economists
and is beyond the scope of this text.
Stable Versus Dynamic
The shapes of the demand patterns for some products or services change over time,
whereas others do not. Those that retain the same general shape are called stable and
those that do not are called dynamic. Dynamic changes can affect the trend, seasonality,
or randomness of the actual demand. The more stable the demand, the easier it is to forecast. Figure 8.3 shows a graphical representation of stable and dynamic demand. Notice
the average demand is the same for both stable and dynamic patterns. It is usually the
average demand that is forecast.
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Chapter eight
Dynamic
Stable
Average
Demand
Figure 8.3 Stable and dynamic demand.
Dependent Versus Independent Demand
Chapter 4 discussed dependent and independent demand, where it was stated that demand
for a product or service is independent when it is not related to the demand for any other
product or service, or independent of internal activities of the firm. Dependent demand for
a product or service occurs where the demand for the item is derived from that of a second
item. Requirements for dependent demand items need not be forecast but are calculated
from that of the independent demand item.
Only independent demand items need to be forecasted. These are usually end items or
finished goods but should also include service parts and items supplied to other plants in
the same company (intercompany transfers).
PrinciPles oF Forecasting
Forecasts have four major characteristics or principles. An understanding of these will allow
the more effective use of forecasts. They are simple and, to some extent, common sense.
1. Forecasts are usually wrong. Forecasts attempt to look into the unknown future
and, except by sheer luck, will be wrong to some degree. Errors are inevitable and
must be expected.
2. Every forecast should include an estimate of error. Since forecasts are expected
to be wrong, the real question is “by how much?” Every forecast should include an
estimate of error often expressed as a percentage (plus and minus) of the forecast or as
a range between maximum and minimum values. Estimates of this error can be made
statistically by studying the variability of demand about the average demand.
3. Forecasts are more accurate for families or groups. The behavior of individual items
in a group is random even when the group has very stable characteristics. For example,
the marks for individual students in a class are more difficult to forecast accurately than
the class average. High marks average out with low marks. This means that forecasts are
more accurate for large groups of items than for individual items in a group.
For production planning, families or groups are based on the similarity of process and
equipment used. For example, a firm forecasting the demand for knit socks as a product
group might forecast men’s socks as one group and women’s as another since the markets
are different. However, production of men’s and women’s ankle socks will be done on the
same machines and knee socks on another. For production planning, the forecast should
be for (a) men’s and women’s ankle socks and (b) men’s and women’s knee socks.
4. Forecasts are more accurate for nearer time periods. The near future holds less
uncertainty than the far future. Most people are more confident in forecasting what
they will be doing over the next week than a year from now. As someone once said,
tomorrow is expected to be pretty much like today.
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195
In the same way, demand for the near term is easier for a company to forecast
than for a time in the distant future. This is extremely important for long lead time
items and especially so if their demand is dynamic. Anything that can be done to reduce lead time will improve forecast accuracy.
collection and PreParation oF data
Forecasts are usually based on historical data manipulated in some way using either judgment or a statistical technique. Thus, the forecast is only as good as the data on which it is
based. To get good data, three principles of data collection are important.
1. Record data in the same terms as needed for the forecast. There is often a problem
in determining the purpose of the forecast and what is to be forecast. There are three
dimensions to this:
a. If the purpose is to forecast demand on production, data based on demand, not
shipments, are needed. Shipments show when goods were shipped and not necessarily when the customer wanted them. Thus, shipments do not necessarily give a
true indication of demand.
b. The forecast period, in weeks, months, or quarters, should be the same as the
schedule period. If schedules are weekly, the forecast should be for the same time
interval.
c. The items forecasted should be the same as those controlled by manufacturing.
For example, if there are a variety of options that can be supplied with a particular
product, the demand for the product and for each option should be forecast.
Suppose a firm makes a bicycle that comes in three frame sizes, three possible wheel
sizes, a 3-, 5-, or 10-speed gear changer, and with or without deluxe trim. In all, there
are 54 13 * 3 * 3 * 22 individual end items sold. If each were forecast, there would
be 54 forecasts to make. A better approach is to forecast (a) total demand and (b) the
percentage of the total that requires each frame size, wheel size, and so on. That way
there need be only 12 forecasts: 3 frame sizes, 3 wheel sizes, 3 gear changers, 2 levels
of trim, and the bike itself.
In this example, the lead time to make the components would be relatively long
in comparison to the lead time to assemble a bike. Manufacturing can make the components according to component forecast and can then assemble bikes according to
customer orders. This would be ideal for situations where final assembly schedules
are used as discussed in Chapter 3.
2. Record the circumstances relating to the data. Demand is influenced by particular events, and these should be recorded along with the demand data. For
instance, artificial bumps in demand can be caused by sales promotions, price
changes, changes in the weather, or a strike at a competitor’s factory. It is vital that
these factors be related to the demand history so they may be included or removed
for future conditions.
3. Record the demand separately for different customer groups. Many firms distribute their goods through different channels of distribution, each having its own
demand characteristics. For example, a firm may sell to a number of wholesalers that
order relatively small quantities regularly and also sell to a major retailer that buys a
large lot twice a year. Forecasts of average demand would be meaningless, and each
set of demands should be forecast separately.
Forecasting techniques
There are many forecasting methods, but they can usually be classified into categories.
Forecasting techniques generally may be qualitative or quantitative, and they can be based
on extrinsic (external) or intrinsic (internal) factors (APICS Dictionary, 14th edition).
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Qualitative Techniques
Qualitative techniques are projections based on judgment, intuition, and informed opinions. By their nature, they are subjective. Such techniques are used to forecast general
business trends and the potential demand for large families of products over an extended
period of time. As such, they are used mainly by senior management. Production and
inventory forecasting is usually concerned with the demand for particular end items, and
qualitative techniques are seldom appropriate.
When attempting to forecast the demand for a new product, there is no history on
which to base a forecast. In these cases, the techniques of market research and historical
analogy might be used. Market research is a systematic, formal, and conscious procedure for testing to determine customer opinion or intention. Historical analogy is based
on a comparative analysis of the introduction and growth of similar products in the hope
that the new product behaves in a similar fashion. Another method is to test-market a
product.
There are several other methods of qualitative forecasting. An example of one such
method is the Delphi method. This method uses a panel of experts who give their opinions
on what is likely to happen.
Quantitative Techniques
Quantitative techniques are projections based on historical or numerical data, whether it
be from inside or outside the organization.
Extrinsic Techniques
Extrinsic forecasting techniques are projections based on external (extrinsic) indicators
that relate to the demand for a company’s products. Examples of such data would be housing starts, birth rates, and disposable income. The theory is that the demand for a product
group is directly proportional, or correlates, to activity in another field. Examples of correlation follow:
■■
■■
Sales of bricks are proportional to housing starts.
Sales of automobile tires are proportional to gasoline consumption.
Housing starts and gasoline consumption are called economic indicators. They
describe economic conditions prevailing during a given time period. Because these
indicators occur earlier in time than what they help to indicate, they are often called
leading indicators. As an example, housing starts (often indicated by the requests for
building permits) will lead to the need for housing materials, including roofing materials, electrical materials, and so on. Housing starts are, therefore, a good leading indicator for demand for many related housing materials. Some commonly used economic
indicators are construction contract awards, automobile production, farm income, steel
production, and gross national income. Data of this kind is compiled and published by
various government departments, financial papers and magazines, trade associations,
and banks.
The problem is to find an indicator that correlates with demand and one that preferably
leads demand, that is, one that occurs before the demand does. For example, the number of
construction contracts awarded in one period may determine the building material sold in
the next period. When it is not possible to find a leading indicator, it may be possible to use
a non-leading indicator for which the government or an organization forecasts. In a sense,
it is basing a forecast on a forecast.
Extrinsic forecasting is most useful in forecasting the total demand for a firm’s products or the demand for families of products. As such, it is used most often in business and
production planning rather than the forecasting of individual end items.
Forecasting and Demand Management
197
Intrinsic Techniques
Intrinsic forecasting techniques use historical data to forecast. This data is usually
recorded in the company and is readily available. Intrinsic forecasting techniques are
based on the assumption that what happened in the past will happen in the future. This
assumption has been likened to driving a car by looking out the rearview mirror. Although
there is some obvious truth to this, it is also true that lacking any other crystal ball, the best
guide to the future is what has happened in the past.
Since intrinsic techniques are so important, the next section will discuss some of the
more important intrinsic techniques. They are often used as input to master production
scheduling where end item forecasts are needed for the planning horizon of the plan.
some imPortant intrinsic techniques
Assume that the monthly demand for a particular item over the past year is as shown in
Figure 8.4.
Suppose it is the end of December, and a forecast is needed for demand for January of
the coming year. Several rules can be used:
Demand this month will be the same as last month. January demand would be
forecast at 84, the same as December. This may appear too simple, but if there is little
change in demand month to month, it probably will be quite usable.
■■ Demand this month will be the same as demand the same month last year.
Forecast demand would be 92, the same as January last year. This rule is adequate if
demand is seasonal and there is little up or down trend.
■■
Rules such as these, based on a single month or past period, are of limited use when
there is much random fluctuation in demand. Usually methods that average out history are
better because they dampen out some effects of random variation.
As an example, the average of last year’s demand can be used as an estimate for
January demand. Such a simple average would not be responsive to trends or changes in
level of demand. A better method would be to use a moving average.
Average demand This raises the question of what to forecast. As discussed previously, demand can fluctuate because of random variation. It is best to forecast the average
demand rather than second-guess what the effect of random fluctuation will be. The second
principle of forecasting discussed previously said that a forecast should include an estimate
of error. This range can be estimated, so a forecast of average demand should be made, and
the estimate of error applied to it.
Moving Averages
One simple way to forecast is to take the average demand for, say, the last three or six periods and use that figure as the forecast for the next period. At the end of the next period, the
first period demand is dropped and the latest period demand added to determine a new average to be used as a forecast. This forecast would always be based on the average of the actual
demand over the specified period.
January
February
March
April
May
June
92
83
66
74
75
84
Figure 8.4 A 12-month demand history.
July
August
September
October
November
December
84
81
75
63
91
84
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Chapter eight
For example, suppose it was decided to use a three-month moving average on the
data shown in Figure 8.4. The forecast for January, based on the demand in October,
November, and December, would be:
63 + 91 + 84
= 79
3
Now suppose that January demand turned out to be 90 instead of 79. The forecast for
February would be calculated as:
91 + 84 + 90
= 88
3
example Problem
Demand over the past three months has been 120, 135, and 114 units. Using a
three-month moving average, calculate the forecast for the fourth month.
Answer
Forecast for month 4 =
120 + 135 + 114
369
=
= 123
3
3
Actual demand for the fourth month turned out to be 129. Calculate the forecast
for the fifth month.
Forecast for month 5 =
135 + 114 + 129
= 126
3
In the previous discussion, the forecast for January was 79, and the forecast for February
was 88. The forecast has risen, reflecting the higher January value and the dropping of the
low October value. If a longer period, such as six months, is used, the forecast does not react
as quickly. The fewer months included in the moving average, the more weight is given to
the latest information, and the faster the forecast reacts to trends. However, the forecast will
always lag behind a trend. For example, consider the following demand history for the past
five periods:
Period
1
2
3
4
5
Demand
1000
2000
3000
4000
5000
There is a rising trend to demand. If a five-period moving average is used,
the forecast for period 6 is 11000 + 2000 + 3000 + 4000 + 50002 , 5 = 3000.
It does not look very accurate since the forecast is lagging actual demand by a
large amount. However, if a three-month moving average is used, the forecast is
13000 + 4000 + 50002 , 3 = 4000. Not perfect, but somewhat better. The point is
that a moving average always lags a trend, and the more periods included in the average,
the greater the lag will be.
On the other hand, if there is no trend but actual demand fluctuates considerably due
to random variation, a moving average based on a few periods reacts to the fluctuation
rather than forecasts the average. Consider the following demand history:
Period
1
2
3
4
5
Demand
2000
5000
3000
1000
4000
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199
The demand has no trend and is random. If a five-month moving average is used, the
forecast for the next month is 3000. This reflects all the values. If a two-month average is
taken, the forecasts for the third, fourth, fifth, and sixth months are as follows:
Forecast for third month = 12000 + 50002 , 2 = 3500
Forecast for fourth month = 15000 + 30002 , 2 = 4000
Forecast for fifth month = 13000 + 10002 , 2 = 2000
Forecast for sixth month = 11000 + 40002 , 2 = 2500
With a two-month moving average, the forecast reacts very quickly to the latest
demand and thus is not stable.
Moving averages are best used for forecasting products with stable demand where
there is little trend or seasonality. Moving averages are also useful to filter out random
fluctuations. This has some common sense since periods of high demand are often followed by periods of low demand since the total market demand is usually constant and
consumers often buy goods ahead of time due to sales or other outside influences, including the weather and holiday events. Buying early will lower future sales.
One drawback to using moving averages is the need to retain several periods of history for each item to be forecast. This will require a great deal of computer storage or
clerical effort. Also, the calculations are cumbersome. A common forecasting technique,
called exponential smoothing, gives the same results as a moving average but without the
need to retain as much data and with easier calculations.
Exponential Smoothing
Using exponential smoothing, it is not necessary to keep months of history to get a moving average because the previously calculated forecast has already allowed for this history.
Therefore, the forecast can be based on the old calculated forecast and the new data.
Using the data in Figure 8.4, suppose an average of the demand of the last six
months (80 units) is used to forecast January demand. If at the end of January, actual
demand is 90 units, July’s demand is dropped and January’s demand is used to determine the new forecast. However, if an average of the old forecast (80) and the actual
demand for January (90) is taken, the new forecast, for February, is 85 units. This formula puts as much weight on the most recent month as on the old forecast (all previous
months). If this does not seem suitable, less weight could be put on the latest actual
demand and more weight on the old forecast. Perhaps putting only 10% of the weight
on the latest month’s demand and 90% of the weight on the old forecast would be better. In that case,
February forecast = 0.11902 + 0.91802 = 81
Notice that this forecast did not rise as much as the previous calculation in which the
old forecast and the latest actual demand were given the same weight. One advantage to
exponential smoothing is that the new data can be given any weight wanted.
The weight given to latest actual demand is called a smoothing constant and is represented by the Greek letter alpha (a). It is always expressed as a decimal and typically
ranges from 0 to 0.3.
In general, the formula for calculating the new forecast is as follows:
New forecast = (a) (latest demand) + (1 - a) (previous forecast)
example Problem
The old forecast for May was 220, and the actual demand for May was 190. If alpha (a)
is 0.15, calculate the forecast for June. If June demand turns out to be 218, calculate
the forecast for July.
Answer
June forecast = 10.15211902 + 11 - 0.15212202 = 215.5
July forecast = 10.15212182 + 10.8521215.52 = 215.9
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Chapter eight
140
130
Actual
Demand
DEMAND
120
110
100
Forecast: α = .1
90
80
1
2
3
4
5
PERIODS
Forecast: α = .3
6
7
8
Figure 8.5 Exponential forecast where trend exists.
Exponential smoothing provides a routine method for regularly updating item forecasts.
It works quite well when dealing with stable items. Generally, it has been found satisfactory
for short-range forecasting. It is not satisfactory where the demand is low or intermittent.
Exponential smoothing will detect trends, although the forecast will lag actual
demand if a definite trend exists. Figure 8.5 shows a graph of the exponentially smoothed
forecast lagging the actual demand where a positive trend exists. Notice the forecast with
the larger α follows actual demand more closely.
If a trend exists, it is possible to use a slightly more complex formula, called double
exponential smoothing. This technique uses the same principles but notes whether each
successive value of the forecast is moving up or down on a trend line. Double exponential
smoothing is beyond the scope of this text.
A problem exists in selecting the “best” alpha factor. If a low factor such as 0.1 is
used, the old forecast will be heavily weighted, and changing trends will not be picked up
as quickly as might be desired. If a larger factor such as 0.4 is used, the forecast will react
sharply to changes in demand and will be erratic if there is a sizable random fluctuation.
A good way to get the best alpha factor is to use computer simulation. Using past actual
demand, forecasts are made with different alpha factors to see which one best suits the
historical demand pattern for particular products.
seasonality
Many products have a seasonal or periodic demand pattern: skis, lawnmowers, bathing
suits, and Christmas tree lights are examples. Less obvious are products whose demand
varies by the time of day, week, or month. Examples of these might be electric power
usage during the day or grocery shopping during the week. Power usage peaks between
4:00 P.M. and 7:00 P.M. and supermarkets are most busy toward the end of the week or
before certain holidays.
Seasonal Index
A useful indication of the degree of seasonal variation for a product is the seasonal index.
This index is an estimate of how much the demand during the season will be above or
below the average demand for the product. For example, swimsuit demand might average
100 per month, but in July the average could be 175, and in September it could be 35. The
index for July demand would be 1.75, and for September it would be 0.35.
Forecasting and Demand Management
Year
1
122
130
132
128
1
2
3
Average
Quarter
3
81
73
71
75
2
108
100
98
102
4
90
96
99
95
201
Total
401
399
400
400
140
130
Average Demand
DEMAND
120
110
100
90
80
70
60
Actual Demand
1
2
3
4
5
6
7
QUARTERS
8
9
10
11
12
Figure 8.6 Seasonal sales history.
The formula for the seasonal index is as follows:
Seasonal index =
period average demand
average demand for all periods
The period can be daily, weekly, monthly, or quarterly depending on the basis for the
seasonality of demand.
The average demand for all periods is a value that averages out seasonality. This is
called the deseasonalized demand. The previous equation can be rewritten as follows:
Deseasonalized demand =
period average demand
seasonal index
example Problem
A product that is seasonally based on quarterly demand and the demand for the past
three years is shown in Figure 8.6. There is no trend, but there is definite seasonality.
Average quarterly demand is 100 units. Figure 8.6 also shows a graph of actual
seasonal demand and average quarterly demand. The average demand shown is the
historical average demand for all periods. Remember, this is a forecast of average
demand, not seasonal demand.
Answer
The seasonal indices can now be calculated as follows:
Seasonal index =
128
= 1.281quarter 12
100
=
102
= 1.021quarter 22
100
=
75
= 0.751quarter 32
100
=
95
= 0.951quarter 42
100
Total of seasonal indices = 4.00
Note that the total of all the seasonal indices equals the number of periods. This is a
good way to check whether the calculations are correct.
Chapter eight
Seasonal Forecasts
The equation for developing seasonal indices is also used to forecast seasonal demand. If
a company forecasts average demand for all periods, the seasonal indices can be used to
calculate the seasonal forecasts. Changing the equation around results in:
Seasonal demand = 1seasonal index21average demand2
actual seasonal demand
Deseasonalized demand =
seasonal index
example Problem
The company in the previous problem forecasts an annual demand next year of 420 units.
Calculate the forecast for quarterly sales.
Answer
Expected quarter demand = 1seasonal index21average quarterly demand2
Expected first - quarter demand = 1.28 * 105 = 134.4 units
Expected second - quarter demand = 1.02 * 105 = 107.1 units
Expected third - quarter demand = 0.75 * 105 = 78.75 units
Expected fourth - quarter demand = 0.95 * 105 = 99.75 units
Total forecast demand =
420
units
Deseasonalized Demand
Forecasts do not consider random variation. They are made for average demand, and
seasonal demand is calculated from the average using seasonal indices. Figure 8.7 shows
both actual demand and forecast average demand. The forecast average demand is also
the deseasonalized demand. Historical data is of actual seasonal demand, and it must be
deseasonalized before it can be used to develop a forecast of average demand.
Also, if comparisons are made between sales in different periods, they are meaningless unless deseasonalized data is used. For example, a company selling tennis rackets
finds demand is usually largest in the summer. However, some people play indoor tennis,
so there is demand in the winter months as well. If demand in January was 5200 units and
in June was 24,000 units, how could January demand be compared to June demand to see
25
20
DEMAND
202
Forecast Demand
15
10
Actual Demand
5
0
1
2
3
4
5
6
7
QUARTERS
Figure 8.7 Seasonal demand.
8
9
10
11
12
Forecasting and Demand Management
203
which was the better demand month? If there is seasonality, comparison of actual demand
would be meaningless. Deseasonalized data is needed to make a comparison.
The equation to calculate deseasonalized demand for each period is derived from the
previous seasonal equation and is as follows:
Deseasonalized demand =
actual seasonal demand
seasonal index
example Problem
A company selling tennis rackets has a January demand of 5200 units and a June
demand of 24,000 units. If the seasonal indices for January were 0.5 and for June
were 2.5, calculate the deseasonalized January and June demand. How do the two
months compare?
Answer
Deseasonalized January demand = 5200 , 0.5 = 10,400 units
Deseasonalized June demand = 24,000 , 2.5 = 9600 units
June and January demand can now be compared. On a deseasonalized basis, January
demand is greater than June demand.
Deseasonalized data must be used for forecasting. Forecasts are made for average
demand, and the forecast for seasonal demand is calculated from the average demand
using the appropriate season index.
The rules for forecasting with seasonality follow:
Use only deseasonalized data to forecast.
Forecast deseasonalized demand (base forecast), not seasonal demand.
■■ Calculate the seasonal forecast by applying the seasonal index to the base forecast.
■■
■■
example Problem
A company uses exponential smoothing to forecast demand for its products. For April,
the deseasonalized forecast was 1000, and the actual seasonal demand was 1250 units.
The seasonal index for April is 1.2 and for May is 0.7. If a is 0.1, calculate the following:
a. The deseasonalized actual demand for April.
b. The deseasonalized May forecast.
c. The seasonal forecast for May.
Answer
a. Deseasonalized actual demand for April =
1250
= 1042
1.2
b. Deseasonalized May forecast = a(latest actual) + (1 - a) (previous forecast)
= 0.1110422 + 0.9110002 = 1004
c. Seasonalized May forecast = (seasonal index)(deseasonalized forecast)
= 0.7110042 = 703
tracking the Forecast
As noted in the discussion on the principles of forecasting, forecasts are usually wrong.
There are several reasons for this, some of which are related to human involvement and
others to the behavior of the economy and customers. If there were a method of determining how good a forecast is, forecasting methods could be improved and better estimates
could be made accounting for the error. There is no point in continuing with a plan based
on poor forecast data, so the forecast must be tracked. Tracking the forecast is the process
of comparing actual demand with the forecast.
Chapter eight
Forecast Error
Forecast error is the difference between actual demand and forecast demand. Error can
occur in two ways: bias and random variation.
Bias Cumulative actual demand may not be the same as forecast. Consider the data in
Figure 8.8. Actual demand varies from forecast, and over the six-month period, cumulative demand is 120 units greater than expected.
Bias exists when the cumulative actual demand varies from the cumulative forecast. This means the forecast average demand has been wrong. In the example in
Figure 8.8, the forecast average demand was 100, but the actual average demand was
720 , 6 = 120 units. Figure 8.9 shows a graph of cumulative forecast and actual demand.
Month
Forecast
Actual
Monthly
Cumulative
Monthly
Cumulative
1
100
100
110
110
2
100
200
125
235
3
100
300
120
355
4
100
400
125
480
5
100
500
130
610
6
100
600
110
720
Total
600
600
720
720
Figure 8.8 Forecast and actual sales with bias.
800
Forecast
Actual
700
600
DEMAND
204
500
400
300
200
100
0
0
1
2
3
MONTH
Figure 8.9 Forecast and actual demand with bias.
4
5
6
Forecasting and Demand Management
205
Bias is a systematic error in which the actual demand is consistently above or below
the forecast demand. When bias exists, the forecast should be evaluated and possibly
changed to improve its accuracy.
The purpose of tracking the forecast is to be able to react to forecast error by planning
around it or by reducing it. When an unacceptably large error or bias is observed, it should
be investigated to determine its cause.
Often there are exceptional one-time reasons for error. Examples are competitor
actions, customer shutdown, large one-time orders, and sales promotions. These reasons
relate to the discussion on collection and preparation of data and the need to record the circumstances relating to the data. On these occasions, the demand history must be adjusted
to consider the exceptional circumstances.
Errors can also occur because of timing. For example, an early or late winter will affect
the timing of demand for snow shovels although the cumulative demand will be the same.
Tracking cumulative demand will confirm timing errors or exceptional one-time
events. The following example illustrates this. Note that in April the cumulative demand
is back in a normal range.
Month
Forecast
Actual
Cumulative Forecast
January
100
95
100
February
100
110
200
March*
100
155
300
April
100
45
400
May
100
90
500
*Customer foresaw a possible strike and stockpiled.
Cumulative Actual
95
205
360
405
495
Random variation In a given period, actual demand will vary about the average
demand. The variability will depend upon the demand pattern of the product. Some products will have a stable demand, and the variation will not be large. Others will be unstable
and will have a large variation.
Consider the data in Figure 8.10, showing forecast and actual demand. Notice there is
much random variation, but the average error is zero. This shows that the average forecast
was correct and there was no bias. The data is plotted in Figure 8.11.
Mean Absolute Deviation
Forecast error must be measured before it can be used to revise the forecast or to help in
planning. There are several ways to measure error. One method commonly used due to its
ease of calculation is mean absolute deviation (MAD).
Month
Forecast
Actual
Variation
(error)
1
100
105
5
2
100
94
–6
3
100
98
–2
4
100
104
4
5
100
103
3
6
100
96
–4
Total
600
600
0
Figure 8.10 Forecast and actual sales without bias.
Chapter eight
120
115
110
DEMAND
206
Forecast
105
100
95
Actual
90
85
80
1
2
3
4
5
6
MONTHS
Figure 8.11 Forecast and actual sales without bias.
Consider the data on variability in Figure 8.10. Although the total error (variation)
is zero, there is still considerable variation each month. Total error would be useless to
measure the variation. One way to measure the variability is to calculate the total error
ignoring the plus and minus signs and take the average. This is mean absolute deviation:
mean implies an average,
absolute means without reference to plus and minus,
■■ deviation refers to the error:
■■
■■
MAD =
sum of absolute deviations
number of observations
example Problem
Given the data shown in Figure 8.10, calculate the mean absolute deviation.
Answer
Sum of absolute deviations = 5 + 6 + 2 + 4 + 3 + 4 = 24
MAD =
24
= 4
6
Notice that if the deviations are not taken as absolute numbers, the result is quite
different:
Sum of deviations = 5 + (-6) + (-2) + 4 + 3 + ( -4) = 0
Clearly, this example shows that the actual deviations cannot be used as a good
method to determine average forecast error, for in the above case it would show the “average” forecast error to be zero. The MAD is better for that. What the sum of the deviations
being equal to zero does show is that there is no bias. In general, it can be said that any
non-zero sum of deviations over time indicates forecast bias exists. Since the deviations
are found by subtracting the forecast from the actual demand for a given period, a positive
sum of deviations implies the sum of the actual demands are greater than the sum of the
forecasts and implies the forecasting method is biased on the low side. A negative sum of
deviations, on the other hand, implies the forecasting method is biased on the high side.
This type of knowledge, when taken with the MAD for the data, can provide significant
input into planning for safety inventory (discussed in more detail in Chapter 11).
Forecasting and Demand Management
1%
4%
–3
15%
–2
30%
–1
30%
0
15%
1
1%
4%
2
207
3
MEAN ABSOLUTE DEVIATIONS
Figure 8.12 Normal distribution curve.
Normal distribution The mean absolute deviation measures the difference (error)
between actual demand and forecast. Usually, actual demand is close to the forecast but
sometimes is not. A graph of the number of times (frequency) actual demand is of a particular value produces a bell-shaped curve. This distribution is called a normal distribution and is shown in Figure 8.12. Chapter 11 gives a more detailed discussion of normal
distributions and their characteristics.
There are two important characteristics to normal curves: the central tendency, or
average, and the dispersion, or spread, of the distribution. In Figure 8.12, the central
tendency is the forecast. The dispersion, the fatness or thinness of the normal curve, is
measured by the standard deviation. The greater the dispersion, the larger the standard
deviation. The mean absolute deviation is an approximation of the standard deviation and
is used because it is easy to calculate and apply.
From statistics it has been proven that the error will be within
±1 MAD of the average about 60% of the time;
±2 MAD of the average about 90% of the time;
±3 MAD of the average about 98% of the time.
Uses of mean absolute deviation Mean absolute deviation has several uses. Some
of the most important follow.
Tracking signal Bias exists when cumulative actual demand varies from forecast. The
problem is in guessing whether the variance is due to random variation or bias. If the variation is due to random variation, the error will correct itself, and nothing should be done
to adjust the forecast. However, if the error is due to bias, the forecast should be corrected.
Using the mean absolute deviation, judgment can be made about the reasonableness of
the error. Under normal circumstances, the actual period demand will be within ±3 MAD
of the average 98% of the time. If actual period demand varies from the forecast by more
than 3 MAD, there is about 98% probability that the forecast is in error.
A tracking signal can be used to monitor the quality of the forecast. There are several
procedures used, but one of the simpler ones is based on a comparison of the cumulative
sum of the forecast errors (the cumulative bias) to the mean absolute deviation. Following
is the equation:
Tracking signal =
algebraic sum of forecast errors
MAD
example Problem
The forecast is 100 units a week. The actual demand for the past six weeks has been
105, 110, 103, 105, 107, and 115. If MAD is 7.5, calculate the sum of the forecast
error and the tracking signal.
Answer
Some of forecast error = 5 + 10 + 3 + 5 + 7 + 15 = 45
Tracking signal = 45 , 7.5 = 6
208
Chapter eight
example Problem
A company uses a trigger of ±4 to decide whether a forecast should be reviewed. Given
the following history, determine in which period the forecast should be reviewed. MAD
for the item is 2.
Period
Forecast
Actual
1
100
96
2
100
98
3
100
104
4
100
110
Forecast
Actual
Deviation
Cumulative
Deviation
Tracking
Signal
5
2.5
Cumulative
Deviation
Tracking
Signal
5
2.5
Answer
Period
Deviation
1
100
96
–4
1
0.5
2
100
98
–2
–1
–0.5
3
100
104
4
3
1.5
4
100
110
10
13
6.5
The forecast should be reviewed in period 4.
Contingency planning Suppose a forecast is made that demand for door slammers will be 100 units and that capacity for making them is 110 units. Mean absolute
deviation of actual demand about the forecast historically has been calculated at
10 units. This means there is a 60% chance that actual demand will be between 90 and
110 units and a 40% chance that they will not. With this information, manufacturing
management might be able to devise a contingency plan to cope with the possible
extra demand.
Safety stock The data can be used as a basis for setting safety stock. This will be discussed in detail in Chapter 11.
P/D Ratio
Because of the inherent error in forecasts, companies that rely on them can run into a variety of problems. For example, the wrong material may be bought and perhaps processed
into the wrong goods. A more reliable way of producing what is really needed is the use
of the P/D ratio.
P, or production lead time, is the stacked lead time for a product. It includes time
for purchasing and arrival of raw materials, manufacturing, assembly, delivery, and sometimes the design of the product. Figure 1.1 on page 3 shows various times in different
types of industries and is reproduced in Figure 8.13.
Forecasting and Demand Management
209
Delivery Lead Time
DESIGN
PURCHASE
MANUFACTURE
ASSEMBLE
SHIP
ENGINEER-TOORDER
SHIP
MAKE-TOORDER
SHIP
CONFIGURETO-ORDER
Delivery Lead Time
MANUFACTURE
INVENTORY
ASSEMBLE
Delivery Lead Time
MANUFACTURE
INVENTORY
ASSEMBLE
Delivery Lead Time
MANUFACTURE
ASSEMBLE
SHIP
ASSEMBLETO-ORDER
INVENTORY
SHIP
MAKE-TOSTOCK
INVENTORY
Delivery Lead Time
MANUFACTURE
ASSEMBLE
Figure 8.13 Manufacturing lead time and strategy.
D, or demand lead time, is the customer’s lead time. It is the time from when a customer places an order until the goods are delivered. It can be very short, as in a make-tostock environment, or very long, as in an engineer-to-order company.
The traditional way to guard against inherent error in forecasting is to include safety
stock in inventory. There is an added expense to the extra inventory carried “just in case.”
One other way is to make more accurate predictions. There are five ways to move in this
direction.
1. Reduce P time. The longer the P time, the more chance there is for error. Ideally, P
will be less than D.
2. Force a match between P and D. Moving in this direction can be done in two ways:
a. Make the customer’s D time equal to your P time. This is common with custom products
when the manufacturer makes the product according to the customer’s specification.
b. Sell what you forecast. This will happen while controlling the market. One good
example is the automobile market. It is common to offer special inducements toward
the end of the automotive year in order to sell what the manufacturers have predicted.
3. Simplify the product line. The more variety in the product line, the more room for error.
4. Standardize products and processes. This means that customization must occur
closer to final assembly. This technique is also known as postponement, as discussed
in Chapter 1. The basic components are identical, or similar, for all components.
Figure 8.14 shows this graphically.
5. Forecast more accurately. Make forecasts using a well-thought-out, well-controlled
process.
End Product - Variable
Standard Ingredients
Figure 8.14 Mushroom design.
210
Chapter eight
If P is less than D, it implies the product can be produced in a shorter time period than
the expected customer lead time. Production can be started with actual order information
and still deliver within the customer lead time.
summary
Forecasting is an inexact science that is, nonetheless, an invaluable tool if the following
are kept in mind:
Forecasts should be tracked.
There should be a measure of reasonableness of error.
■■ When actual demand exceeds the reasonableness of error, an investigation should be
made to discover the cause of the error.
■■ If there is no apparent cause of error, the method of forecasting should be reviewed to
see if there is a better way to forecast.
■■
■■
There are several methods used to forecast, including qualitative, quantitative, intrinsic, and extrinsic methods.
key terms
Average demand 197
Bias 204
Collaborative planning, forecasting, and
replenishment (CPFR) 191
Demand lead time 209
Demand management 190
Deseasonalized demand 201
Dynamic 193
Economic indicators 196
Exponential smoothing 199
Extrinsic forecasting techniques 196
Forecast error 204
Intrinsic forecasting techniques 197
Leading indicator 196
Mean absolute deviation (MAD) 205
Moving averages 199
Normal distribution 207
Order processing 192
Production lead time 208
Qualitative techniques 196
Quantitative techniques 196
Random variation 193
Seasonal index 200
Seasonality 193
Smoothing constant 199
Stable 193
Tracking signal 207
Trend 193
questions
1. What is demand management? What functions does it include?
2. Why must we forecast?
3. What factors influence the demand for a firm’s products?
4. Describe the purpose of forecasting for strategic business planning, sales and operations planning, and master production scheduling.
5. The text describes three characteristics of demand. Name and describe each.
6. Describe trend, seasonality, random variation, and cycle as applied to forecasting.
7. The text discusses four principles of forecasting. Name and describe each.
8. Name and describe the three principles of data collection.
Forecasting and Demand Management
211
9. Describe the characteristics and differences between qualitative, quantitative, extrinsic, and
intrinsic forecasting techniques.
10. Describe and give the advantages and disadvantages of (a) moving averages and (b) exponential smoothing.
11. What is a seasonal index? How is it calculated?
12. What is meant by the term deseasonalized demand?
13. What is meant by the term tracking the forecast? In which two ways can forecasts go wrong?
14. What is bias error in forecasting? What are some of the causes?
15. What is random variation?
16. What is the mean absolute deviation (MAD)? Why is it useful in forecasting?
17. What action should be taken when unacceptable error is found in tracking a forecast?
18. What is the P/D ratio? How can it be improved?
19. How would a manufacturer with a P/D ration less than 1 schedule production? How would this
affect inventories?
20. What might it mean if a forecasting method has no bias yet has a large MAD?
Problems
8.1. Over the past three months, the demand for a product has been 255, 219, and 231.
Calculate the three-month moving average forecast for month 4. If the actual demand
in month 4 is 228, calculate the forecast for month 5.
Answer.
235, 226
8.2. Given the following data, calculate the three-month moving average forecasts for
months 4, 5, 6, and 7.
Month
Actual Demand
1
60
2
70
3
40
4
50
5
70
6
65
7
Forecast
212
Chapter eight
8.3. Monthly demand over the past 10 months is given in what follows.
a. Graph the demand.
b. What is your best guess for the demand for month 11?
c. Using a three-month moving average, calculate the forecasts for months 4, 5, 6, 7,
8, 9, 10, and 11.
Month
Actual Demand
1
102
2
91
3
95
4
105
5
94
6
100
7
106
8
95
9
105
10
98
Forecast
11
8.4. If the forecast for February was 122 and actual demand was 135, what would be the
forecast for March if the smoothing constant (a) is 0.15? Use exponential smoothing
for your calculation.
Answer.
Forecast = 123.95 = 124
8.5. If the old forecast is 100 and the latest actual demand is 83, what is the exponentially
smoothed forecast for the next period? Alpha is 0.25.
8.6. Using exponential smoothing, calculate the forecasts for months 2, 3, 4, 5, and 6. The
smoothing constant is 0.2, and the old forecast for month 1 is 245.
Month
Actual Demand
1
260
2
230
3
225
4
245
5
250
6
Forecast Demand
Forecasting and Demand Management
213
8.7. Using exponential smoothing, calculate the forecasts for the same months as in problem 8.3c. The old average for month 3 was 96 and a = 0.4. What is the difference
between the two forecasts for month 11?
Month
Actual Demand
1
102
2
91
3
95
4
105
5
94
6
100
7
106
8
95
9
105
10
98
Forecast
11
8.8. Weekly demand for an item averaged 100 units over the past year. Actual demand for
the next eight weeks is shown in what follows:
a. Plot the data on graph paper.
b. Letting a = 0.25, calculate the smoothed forecast for each week.
c. Comment on how well the forecast is tracking actual demand. Is it lagging or leading actual demand?
Week
Actual Demand
Forecast
1
103
100
2
112
3
113
4
120
5
128
6
131
7
140
8
142
9
214
Chapter eight
8.9. If the average demand for the first quarter was 140 and the average demand for all
quarters was 175, what is the seasonal index for the first quarter?
Answer.
Seasonal index = 0.80
8.10. Using the data in problem 8.9, if the forecast for next year is 800, calculate the forecast for first quarterly demand next year.
Answer.
Forecast for first quarter = 160
8.11. The average demand for January has been 80, and the average annual demand
has been 1800. Calculate the seasonal index for January. If the company forecasts
annual demand next year at 2000 units, what is the forecast for January next year?
8.12. Given the following average demand for each month, calculate the seasonal indices
for each month.
Month
Average Demand
January
30
February
50
March
85
April
110
May
125
June
245
July
255
August
135
September
100
October
90
November
50
December
30
Seasonal Index
Total
Note that your answer, if done correctly, should have all the seasonal indices add up
to the number of periods in the entire season, in this case 12.
Forecasting and Demand Management
215
8.13. Using the data in problem 8.12 and the seasonal indices you have calculated, calculate expected monthly demand if the annual forecast is 2000 units.
Month
Seasonal Index
Forecast
January
February
March
April
May
June
July
August
September
October
November
December
8.14. If the actual demand for April was 1440 units and the seasonal index was 2.5, what
would be the deseasonalized April demand?
Answer.
Deseasonalized demand = 576 units
8.15. Calculate the deseasonalized demands for the following:
Quarter
Actual Demand
Seasonal Index
1
130
0.62
2
170
1.04
3
375
1.82
4
90
0.52
Deseasonalized Demand
Total
8.16. The old deseasonalized forecast is 100 units, a = 30, and the actual demand for the
last month was 180 units. If the seasonal index for the last month is 1.2 and the next
month is 0.8, calculate:
a. The deseasonalized actual demand for the last month.
b. The deseasonalized forecast for next month using exponential smoothing.
c. The forecast of actual demand for the next month.
Answer.
a. Deseasonalized last month’s demand = 150
b. Deseasonalized forecast for next month = 115
c. Forecast of seasonalized demand = 92
216
Chapter eight
8.17. The Fast Track Ski Shoppe sells ski goggles during the four months of the ski season.
Average demand follows:
Month
Average Past Demand
December
300
January
400
February
220
March
130
Seasonal Index
Forecast Demand Next Year
Total
a. Calculate the deseasonalized sales and the seasonal index for each of the four
months.
b. If next year’s demand is forecast at 1200 pairs of goggles, what will be the forecast sales for each month?
8.18. Given the following forecast and actual demand, calculate the mean absolute
deviation.
Period
Forecast
Actual Demand
1
110
85
2
110
105
3
110
120
4
110
100
5
110
90
Absolute Deviation
Total
Answer.
MAD = 14
8.19. For the following data, calculate the mean absolute deviation.
Period
Forecast
Actual Demand
1
100
105
2
105
95
3
110
90
4
115
130
5
120
100
6
125
120
Total
675
650
Absolute Deviation
Forecasting and Demand Management
217
8.20. A company uses a tracking signal trigger of ±4 to decide whether a forecast should
be reviewed. Given the following history, determine in which period the forecast
should be reviewed. MAD for the item is 15. Is there any previous indication that the
forecast should be reviewed?
Period
Forecast
Actual
1
100
110
2
105
90
3
110
85
4
115
110
5
120
105
6
125
95
Deviation
Cumulative Deviation
Tracking Signal
case study 8.1
Northcutt Bikes: The Forecasting Problem
Jan Northcutt, owner of Northcutt Bikes, started business in 1995. She noticed the quality
of bikes she purchased for sale in her bike shop declining while the prices went up. She
also found it more difficult to obtain the features she wanted on ordered bikes without
waiting for months. Her frustration turned to a determination to build her own bikes to her
particular customer specifications.
She began by buying all the necessary parts (frames, seats, tires, etc.) and assembling
them in a rented garage using two helpers. As the word spread about her shop’s responsiveness to options, delivery, and quality, however, the individual customer base grew
to include other bike shops in the area. As her business grew and demanded more of her
attention, she soon found it necessary to sell the bike shop itself and concentrate on the
production of bikes from a fairly large leased factory space.
As the business continued to grow, she backward integrated more and more processes
into her operation, so that now she purchases less than 50% of the component value of
the manufactured bikes. This not only improves her control of production quality but also
helps her control the costs of production and makes the final product more cost attractive
to her customers.
the Current Situation
Jan considers herself a hands-on manager and has typically used her intuition and her
knowledge of the market to anticipate production needs. Since one of her founding principles was rapid and reliable delivery to customer specification, she felt she needed to begin
production of the basic parts for each particular style of bike well in advance of demand.
In that way she could have the basic frame, wheels, and standard accessories started in
production prior to the recognition of actual demand, leaving only the optional add-ons to
assemble once the order came in. Her turnaround time for an order of less than half the industry average is considered a major strategic advantage, and she feels it is vital for her to
maintain or even improve on response time if she is to maintain her successful operation.
As the customer base has grown, however, the number of customers Jan knows personally has shrunk significantly as a percentage of the total customer base for Northcutt Bikes,
and many of these new customers are expecting or even demanding very short response
times, as that is what attracted them to Northcutt Bikes in the first place. This condition, in
218
Chapter eight
addition to the volatility of overall demand, has put a strain on capacity planning. She finds
that at times there is a lot of idle time (adding significantly to costs), whereas at other times
the demand exceeds capacity and hurts customer response time. The production facility has
therefore turned to trying to project demand for certain models and actually building a finished goods inventory of those models. This has not proven to be too satisfactory, as it has
actually hurt costs and some response times. Reasons include the following:
The finished goods inventory is often not the “right” inventory, meaning shortages for
some goods and excessive inventory of others. This condition both hurts responsiveness and increases inventory costs.
■■ Often, to help maintain responsiveness, inventory is withdrawn from finished goods
and reworked, adding to product cost.
■■ Reworking inventory uses valuable capacity for other customer orders, again resulting
in poorer response times and/or increased costs due to expediting. Existing production
orders and rework orders are both competing for vital equipment and resources during
times of high demand, and scheduling has become a nightmare.
■■
The inventory problem has grown to the point that additional storage space is needed,
and that is a cost that Jan would like to avoid if possible.
Another problem Jan faces is the volatility of demand for bikes. Since she is worried
about unproductive idle time and yet does not wish to lay off her workers during times of
low demand, she has allowed them to continue to work steadily and build finished goods.
This makes the problem of building the “right” finished goods even more important, especially given the tight availability of storage space.
past Demand
The following shows the monthly demand for one major product line: the standard
26-inch 10-speed street bike. Although it is only one of Jan’s products, it is representative
of most of the major product lines currently being produced by Northcutt Bikes. If Jan can
find a way to use this data to more constructively understand her demand, she feels she
can probably use the same methodologies to project demand for other major product families. Such knowledge can allow her, she feels, to plan more effectively and continue to be
responsive while still controlling costs.
Month
January
February
March
April
May
June
July
August
September
October
November
December
ACTUAL DEMAND
2011
2012
2013
437
712
613
605
732
984
722
829
812
893
992
1218
901
1148
1187
1311
1552
1430
1055
927
1392
975
1284
1481
822
1118
940
893
737
994
599
983
807
608
872
527
2014
701
1291
1162
1088
1497
1781
1843
839
1273
912
996
792
assignment
1. Plot the data and describe what you see. What does it mean and how would you use
the information from the plot to help you develop a forecast?
2. Use at least two different methodologies to develop as accurate a forecast as possible
for the demand. Use each of those methods to project the next four months demand.
Forecasting and Demand Management
219
3. Which method from question 2 is “better”? How do you know that?
4. How, if at all, could we use Jan’s knowledge of the market to improve the forecast?
Would it be better to forecast in quarterly increments instead of monthly? Why or
why not?
5. Are there other possible approaches that might improve Jan’s operation and situation?
What would they be and how could they help?
6. Has Jan’s operation grown too large for her to control well? Why or why not? What
would you suggest she do? What additional information would you suggest she look
for to help her situation?
case study 8.2
Hatcher gear Company
Jack Fielding found himself in a real dilemma when the sales and marketing department
presented him with the annual sales demand forecast for one of the gear lines. Jack, as
materials manager for Hatcher Gears, was responsible for taking the forecast and translating it into projected production needs and, as part of that, requirements for raw material
purchasing needs—both in quantities and dates. That fairly routine task went fine until he
came to the V27 family of gears.
The V27 family of gears was used by customers in highly stressful applications, and
as a result they needed to be made from a highly specialized steel, made with a complex
mixture of chemicals. The steel mill made the steel reluctantly, since it required shutting
down a furnace and completely cleaning it out in order to avoid contamination. The time
and effort to make the furnace ready, in combination with the costly chemicals used to
make the steel, meant that the steel was extremely expensive for Hatcher to buy. In addition, the steel company had told Hatcher Gear that they would make only one batch of
the steel and only once a year. They had their own annual production plans to execute,
and the special steel was simply too disruptive to their production for Hatcher Gears to
request any additional steel beyond the one batch during the year. Since Hatcher Gears
was the only customer that needed that steel, the steel company required Hatcher to buy
all the steel that was made in the batch. The steel company had no desire to maintain a
very expensive inventory for Hatcher Gear. As it was, the steel company had reluctantly
kept Hatcher Gears as a customer, and his recent attempts to find another steel company
willing to make the steel was met with rapid and emphatic refusals.
Those facts meant that Jack must figure out how much steel to buy as accurately as
he could—whatever amount he bought had to last him for the remainder of the year. If he
bought too little, he might not be able to supply some of the customers for the gears, and
that could be disastrous for customer service, and his boss (the general manager) would
certainly hold him responsible. In fact, just recently the general manager had again emphasized that the customers for V27 gears were important enough that he wanted to have
enough of the steel to be able to supply those customers on time at least 97% of the time,
even though the inventory to do that was certainly very expensive. All of their customers for the V27 gears used Hatcher as their single source of supply. But buying too much
would also be bad. Jack, being materials manager, essentially “owned” the inventory in
the facility, meaning that he was held responsible for all raw, in-process inventory, and
finished goods inventory, including having to answer for the cost of holding that inventory. Having too much of that expensive steel over the year could get him in a lot of trouble with the chief financial officer and the general manager. While the customers for the
gears knew the steel was expensive and they were therefore willing to pay a higher price,
they were not willing to accept a price increase to help Hatcher pay for excessive inventory. They expected Hatcher to manage that.
Knowing all that, Jack’s dilemma was based on looking at the annual sales demand
forecast developed by marketing for the V27 gear family of 16,000 gears. When he got
that forecast for 16,000, he looked at the sales for the V27 over the last ten years:
220
Chapter eight
Ten years ago—9733
Nine years ago—10,115
Eight years ago—9814
Seven years ago—10,033
Six years ago—10,077
Five years ago—9782
Four years ago—10,145
Three years ago—10,097
Two years ago—9924
Last year—9897
Jack decided to call the director of sales and marketing (Phil Johnson) and ask about
the forecast. This is how the conversation went:
jack: “I wanted to ask about the demand forecast for the V27 gear line. You said it
was 16,000. How did you come up with that number?”
phil: “Because that is what we plan to sell.”
jack: “Do you have or expect to have any new customers for the V27 line?”
phil: “No.”
jack: “Do any of your customers have any new applications that use the V27 gears?”
phil: “Not that I know of.”
jack: “Do any of your customers have or expect to have new customers for their
products that use our V27 gears?”
phil: “Not that I know of.”
jack: “Are any of your customers experiencing growth in their products that use V27
gears?”
phil: “Not that I know of”.
jack: “Well then why do you think you will sell 16,000.”
phil: “Because that is what we plan to sell—and you hadn’t better disappoint any of
our customers or be late with any of their orders!”
Jack hung up the phone and seriously wondered what he was to do. If he ended up
with too much inventory or disappointed any customers, he knew the general manager
would blame him. He knew it could be a big mistake to try to put the blame back on the
director of sales and marketing because not only was the director of sales and marketing
at a higher level in the company, but he was also a personal friend of the general manager.
Jack knew they often played golf together on weekends and that their families frequently
attended social events together.
assignment
1. Try to help Jack out with a short-term solution. Help him come up with how much
steel to buy—enough to cover production of how many V27 gears over the next year?
After you come up with a number, justify your solution. Be as comprehensive as
possible in the justification—include all possible options, discussing why you reject
those you reject and why you accepted your recommendation.
2. Develop a recommendation for Jack and the Hatcher Gear Company to use in the long
run, specifically to try to minimize the current dilemma from occurring again.
Chapter
nine
inventory Fundamentals
introduction
Inventories are materials and supplies that a business or institution carries either for sale
or to provide inputs or supplies to the production process. All businesses and institutions
require inventories. Often they are a substantial part of total assets.
Financially, inventories are very important to manufacturing companies. On the
balance sheet, they usually represent from 20 to 60% of total assets. As inventories
are used, their value is converted into cash, which improves cash flow and return on
investment. There is a cost for carrying inventories, which increases operating costs and
decreases profits. Good inventory management is essential.
Inventory management is responsible for planning and controlling inventory from
the raw material stage to the customer. Since inventory either results from production or
supports it, the two cannot be managed separately and, therefore, must be coordinated.
Inventory must be considered at each of the planning levels and is thus part of production
planning, master production scheduling, and material requirements planning. Production
planning is concerned with overall inventory, master planning with end items, and material requirements planning with component parts and raw material.
aggregate inventory management
Aggregate inventory management deals with managing inventories according to their
classification (raw material, work-in-process, and finished goods) and the function they
perform rather than at the individual item level. It is financially oriented and is concerned
with the costs and benefits of carrying the different classifications of inventories. As such,
aggregate inventory management involves:
Flow and kinds of inventory needed.
Supply and demand patterns.
■■ Functions that inventories perform.
■■ Objectives of inventory management.
■■ Costs associated with inventories.
■■
■■
item inventory management
Inventory is managed not only at the aggregate level but also at the item level. Management
must establish decision rules about inventory items so the staff responsible for inventory
control can do their job effectively. These rules include the following:
Which individual inventory items are most important.
How individual items are to be controlled.
■■ How much to order at one time.
■■ When to place an order.
■■
■■
221
222
Chapter nine
This chapter will study aggregate inventory management and some factors influencing inventory management decisions, which include the following:
Types of inventory based on the flow of material.
Supply and demand patterns.
■■ Functions performed by inventory.
■■ Objectives of inventory management.
■■ Inventory costs.
■■
■■
Finally, this chapter will conclude with a study of the first two decisions, deciding the
importance of individual end items and how they are controlled. Subsequent chapters will
discuss the question of how much stock to order at one time and when to place orders.
inventory and the Flow oF material
There are many ways to classify inventories. One often-used classification is related to
the flow of materials into, through, and out of a manufacturing organization, as shown in
Figure 9.1
Raw materials. These are purchased items received that have not entered the production process. They include purchased materials, component parts, and subassemblies.
■■ Work-in-process (WIP). Raw materials that have entered the manufacturing process
and are being worked on or waiting to be worked on.
■■ Finished goods. The finished products of the production process that are ready to be
sold as completed items. They may be held at a factory or central warehouse or at various points in the distribution system.
■■ Distribution inventories. Finished goods located in the distribution system.
■■
SUPPLIER
SUPPLIER
SUPPLIER
RAW MATERIALS
PURCHASED PARTS
AND
MATERIALS
WORK-IN-PROCESS
FINISHED GOODS
WAREHOUSE
WAREHOUSE
WAREHOUSE
CUSTOMER
DEMAND
CUSTOMER
DEMAND
CUSTOMER
DEMAND
Figure 9.1
Inventories and the flow of materials.
inventory Fundamentals
■■
223
Maintenance, repair, and operating supplies (MROs). Items used in production
that do not become part of the product. These include hand tools, spare parts, lubricants, and cleaning supplies.
Classification of an item into a particular inventory depends on the production environment. For instance, sheet steel or tires are finished goods to the supplier but are raw
materials and component parts to the car manufacturer.
supply and demand patterns
If supply met demand exactly, there would be little need for inventory. Goods could be
made at the same rate as demand, and no inventory would build up. For this situation to
exist, demand must be predictable, stable, and relatively constant over a long time period.
If this is so, manufacturing can produce goods on a line-flow basis, matching production to demand. Using this system, raw materials are fed to production as required, work
flow from one workstation to another is balanced so little work-in-process inventory is
required, and goods are delivered to the customer at the rate the customer needs them.
Flow manufacturing systems were discussed in Chapter 1. Because the variety of products
they can make is so limited, demand has to be large enough to justify economically setting up the system. These systems are characteristic of lean manufacturing, which will be
discussed in Chapter 15.
Demand for most products is neither sufficient nor constant enough to warrant
setting up a line-flow system, and these products are usually made in lots or batches.
Workstations are organized by function, for example, all machine tools in one area, all
welding in another, and assembly in another. Work moves in lots from one workstation to
another as required by the routing. By the nature of the system, inventory will build up in
raw materials, work-in-process, and finished goods.
Functions oF inventories
In batch manufacturing, the basic purpose of inventories is to decouple supply and demand.
Inventory serves as a buffer between:
Supply and demand.
Customer demand and finished goods.
■■ Finished goods and component availability.
■■ Requirements for an operation and the output from the preceding operation.
■■ Parts and materials to begin production and the suppliers of materials.
■■
■■
Based on this, inventories can be classified according to the function they perform.
Anticipation Inventory
Anticipation inventories are built up in anticipation of future demand. For example, they
are created ahead of a peak selling season, a promotion program, vacation shutdown, or
possibly the threat of a strike. They are built up to help level production and to reduce the
costs of changing production rates.
Fluctuation Inventory (Safety Stock)
Fluctuation (buffer) inventory is held to cover random unpredictable fluctuations in supply and demand or lead time. If demand or lead time is greater than forecast, a stockout will
occur. Safety stock is carried to protect against this possibility. Its purpose is to prevent disruptions in manufacturing or deliveries to customers. Safety stock is also called buffer stock
or reserve stock.
224
Chapter nine
Lot-Size Inventory
Items purchased or manufactured in quantities greater than needed immediately create
lot-size inventories. This is to take advantage of quantity discounts; to reduce shipping,
clerical, and setup costs; and in cases where it is impossible to make or purchase items at
the same rate that they will be used or sold. Lot-size inventory is sometimes called cycle
stock. It is the portion of inventory that depletes gradually as customers’ orders come in
and is replenished cyclically when suppliers’ orders are received.
Transportation Inventory
Transportation inventories exist because of the time needed to move goods from one
location to another, such as from a plant to a distribution center or a customer. They are
sometimes called pipeline or movement inventories. The average amount of inventory in
transit is
I =
tA
365
where I is the average annual inventory in transit, t is transit time in days, and A is annual
demand. Notice that the transit inventory does not depend upon the shipment size but on
the transit time and the annual demand. The only way to reduce the inventory in transit,
and its cost, is to reduce the transit time. In addition, since there is a direct relationship
between the amount of inventory in a process and the throughput time (flow time) to move
inventory through a process, there is both a cost and time advantage to reduce the inventory in transit. This concept will be developed more completely in Chapter 11.
example Problem
Delivery of goods from a supplier is in transit for 10 days. If the annual demand is
5200 units, what is the average annual inventory in transit?
Answer
I =
10 * 5200
= 142.5 units
365
The problem can be solved in the same way using dollars instead of units.
Hedge Inventory
Some products, such as minerals and commodities, for example, grains or animal products, are traded on a worldwide market. The price for these products fluctuates according
to world supply and demand. If buyers expect prices to rise, they can purchase hedge
inventory when prices are low. Hedging is complex and beyond the scope of this text.
Maintenance, Repair, and Operating (MRO) Supplies
MROs are items used to support general operations and maintenance but do not become
directly part of a product. They include maintenance supplies, equipment spare parts, and
consumables such as cleaning compounds, lubricants, pencils, and erasers.
In most cases planners (the source of most material purchasing requirements for the
company) do not create orders for purchase of MRO material. Orders are often generated
directly from the maintenance or engineering functions. Some MRO material inventories
are maintained based on probability of need, especially in the case of frequently used materials such as cleaning supplies. Materials used for maintenance can often be determined
from the preventive maintenance schedule for the equipment. This is especially important
for maintenance material that may be too expensive to justify keeping inventory on hand
at all times. Some repair material, even if expensive to keep in inventory, may need to be
held in inventory in cases where the material may have a long lead time or if the lack of the
material may mean a critical piece of equipment would need to remain idle until the material is available. Buffer stocks can be held, established in much the same way as production
inventory Fundamentals
225
inventory (discussed in Chapter 11). Statistical analysis for critical maintenance material
can be done to establish mean time between failures (MTBF) of the part. In that way,
maintenance can be scheduled specifically to minimize the probability of a breakdown of
the equipment, and the purchase of the parts can similarly be synchronized with the maintenance schedule to minimize inventory holding costs. This is very much like the maintenance schedules that automobile companies provide with the purchase of an automobile.
The owner is not obligated to follow the maintenance schedule, but ignoring recommendations will increase the probability of a costly and inconvenient breakdown.
Location of MRO material depends on the needs and policies of the company. In
some cases, MRO material is stocked in the same secure warehouse with production material, but sometimes there is a separate MRO stocking location. In either case, much as with
production material, MRO material often represents a substantial financial cost, and location and count accuracies are often just as important as they are with production material.
objectives oF inventory management
A firm wishing to maximize profit will have at least the following objectives:
Maximum customer service.
Low-cost plant operation.
■■ Minimum inventory investment.
■■
■■
Customer Service
In broad terms, customer service is the ability of a company to satisfy the needs of customers. In inventory management, the term is used to describe the availability of items
when needed and is a measure of inventory management effectiveness. The customer can
be a purchaser, a distributor, another plant in the organization, or the workstation where
the next operation is to be performed.
There are many different ways to measure customer service, each with its strengths
and weaknesses, but there is no one best measurement. Some measures are percentage of
orders shipped on schedule, percentage of line items shipped on schedule, and order-days
out of stock.
Inventories help to maximize customer service by protecting against uncertainty. If a
company could forecast exactly what customers want and when, they could plan to meet
demand with no uncertainty. However, demand and the lead time to get an item are often
uncertain, possibly resulting in stockouts and customer dissatisfaction. For these reasons,
it may be necessary to carry extra inventory to protect against uncertainty. This inventory
is called safety stock and will be discussed in Chapter 11.
Operating Efficiency
Inventories help make a manufacturing operation more productive in four ways:
1. Inventories allow operations with different rates of production to operate separately
and more economically. If two or more operations in a sequence have different rates
of output and are to be operated efficiently, inventories must build up between them.
The inventory purposely used to separate operations to improve operational efficiency is often called decoupling inventory.
2. Chapter 2 discussed production planning for seasonal products in which demand
is nonuniform throughout the year. One strategy discussed was to level production
and build anticipation inventory for sale in the peak periods. This would result in the
following:
■■ Lower overtime costs.
■■ Lower hiring and firing costs.
■■ Lower training costs.
■■ Lower subcontracting costs.
■■ Lower capacity required.
Chapter nine
By leveling production, manufacturing can continually produce an amount equal to
the average demand. The advantage of this strategy is that the costs of changing production levels are avoided. Figure 9.2 shows this strategy.
3. Inventories allow manufacturing to run longer production runs, which result in the
following:
■■ Lower setup costs per item. The cost to make a lot or batch depends upon the
setup costs and the run costs. The setup costs are fixed, but the run costs vary with
the number produced. If larger lots are run, the setup costs are absorbed over a
larger number, and the average (unit) cost is lower.
■■ An increase in production capacity due to production resources being used
at a greater portion of the time for processing as opposed to setup. Time on a
work center is taken up by setup and by run time. Output occurs only when an item
is being worked on and not when setup is taking place. If larger quantities are produced at one time, there are fewer setups required to produce a given annual output, and thus more time is available for producing goods. This is most important
with bottleneck resources. Time lost on setup on these resources is lost throughput
(total production) and lost capacity.
4. Inventories allow manufacturing to purchase in larger quantities, which results in
lower ordering costs per unit and quantity discounts.
But all of this is at a price. The problem is to balance inventory investment with the
following:
1. Customer service. The lower the inventory, the higher the likelihood of a stockout
and the lower the level of customer service. The higher the inventory level, the higher
customer service will be.
2. Costs associated with changing production levels. Excess equipment capacity,
overtime, hiring, training, and layoff costs will all be higher if production fluctuates
with demand.
3. Cost of placing orders. Lower inventories can be achieved by ordering smaller
quantities more often, but this practice results in higher annual ordering costs.
4. Transportation costs. Goods moved in small quantities cost more to move per unit
than those moved in large quantities. However, moving large lots implies higher
inventory.
If inventory is carried, there has to be a benefit that exceeds the costs of carrying that
inventory. Someone once said that the only good reason for carrying inventory beyond
current needs is if it costs less to carry it than not. This being so, the discussion will now
turn to the costs associated with inventory.
DEMAND
226
Demand
Production
J
F
M
A
M
J
TIME
Figure 9.2
Operation leveling.
J
A
S
O
N
D
inventory Fundamentals
227
inventory costs
The following costs are used for inventory management decisions:
Item cost.
Carrying costs.
■■ Ordering costs.
■■ Stockout costs.
■■ Capacity-associated costs.
■■
■■
Item Cost
Item cost is the price paid for a purchased item, which consists of the cost of the item and
any other direct costs associated in getting the item into the plant. These could include
such things as transportation, custom duties, and insurance. The inclusive cost is often
called the landed cost. For an item manufactured in-house, the cost includes direct material, direct labor, and factory overhead. These costs can usually be obtained from either
purchasing or accounting.
Carrying Costs
Carrying costs (sometimes called holding costs) include all expenses incurred by the
firm because of the volume of inventory carried. As inventory increases, so do these costs.
They can be broken down into three categories:
1. Capital costs. Money invested in inventory is not available for other uses and as
such represents a lost opportunity cost. The minimum cost would be the interest lost
by not investing the money at the prevailing interest rate, and it may be much higher
depending on investment opportunities for the firm.
2. Storage costs. Storing inventory requires space, workers, and equipment. As inventory increases, so do these costs.
3. Risk costs. The risks in carrying inventory are as follows:
a. Obsolescence. Loss of product value resulting from a model or style change or
technological development.
b. Damage. Inventory damaged while being held or moved.
c. Pilferage. Goods lost, strayed, or stolen.
d. Deterioration. Inventory that rots or dissipates in storage or whose shelf life is
limited.
What does it cost to carry inventory? Actual figures vary from industry to industry
and company to company. Capital costs may vary depending upon interest rates, the
credit rating of the firm, and the opportunities the firm may have for investment. Storage
costs vary with location and type of storage needed. Risk costs can be very low or can be
close to 100% of the value of the item for perishable goods. The carrying cost is usually
defined as a percentage of the dollar value of inventory per unit of time (usually one year).
Textbooks tend to use a figure of 20–30% in manufacturing industries. This is realistic in
many cases but not with all products. For example, the possibility of obsolescence with
fad or fashion items is high, and the cost of carrying such items is greater.
example Problem
A company carries an average annual inventory of $2,000,000. If it estimates the
cost of capital is 10%, storage costs are 7%, and risk costs are 6%, what does it cost
per year to carry this inventory?
Answer
Total cost of carrying inventory = 10% + 7% + 6% = 23%
Annual cost of carrying inventory = 0.23 * $2,000,000 = $460,000
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Chapter nine
Ordering Costs
Ordering costs are those costs that are associated with placing an order either with the
factory or with a supplier. The cost of placing an order does not depend upon the quantity
ordered. Whether a lot of 10 or 100 is ordered, the costs associated with placing the order
are essentially the same. However, the annual cost of ordering depends upon the number
of orders placed in a year.
Ordering costs include the following:
Production control costs. The annual cost and effort expended in production control
depends on the number of orders placed, not on the quantity ordered. The fewer orders
per year, the less cost. The costs incurred are those of issuing and closing orders,
scheduling, loading, dispatching, and expediting.
■■ Setup and teardown costs. Every time an order is issued, work centers have to set
up to run the order and tear down the setup at the end of the run. These costs do not
depend upon the quantity ordered but on the number of orders placed per year.
■■ Lost capacity cost. Every time an order is placed at a work center, the time taken
to set up is lost as productive output time. This represents a loss of capacity and is
directly related to the number of orders placed. It is particularly important and costly
with bottleneck work centers.
■■ Purchase order cost. Every time a purchase order is placed, costs are incurred to
place the order. These costs include order preparation, follow-up, expediting, receiving, authorizing payment, and the accounting cost of receiving and paying the invoice.
■■ Movement or transportation cost. When an order is placed, material for the order
has to be moved from operation to operation.
■■
The annual cost of ordering depends upon the number of orders placed in a year. This
can be reduced by ordering more at one time, resulting in the placing of fewer orders.
However, this drives up the inventory level and the annual cost of carrying inventory.
example Problem
Given the following annual costs, calculate the average cost of placing one order.
Production control salaries = $60,000
Supplies and operating expenses for production control department = $15,000
Cost of setting up work centers for an order = $120
Orders placed each year = 2000
Answer
Average cost =
=
fixed costs
+ variable cost
number of orders
$60,000 + $15,000
+ $120 = $157.50
2000
Stockout Costs
If demand during the lead time exceeds forecast, a stockout can be expected. A stockout can potentially be expensive because of backorder costs, lost sales, and possibly lost
customers. Stockouts can be reduced by carrying extra inventory to protect against those
times when the demand during lead time is greater than forecast.
Capacity-Associated Costs
When output levels must be changed, there may be costs for overtime, hiring, training,
extra shifts, and layoffs. These capacity-associated costs can be avoided by leveling production, that is, by producing items in slack periods for sale in peak periods. However, this
builds inventory in the slack periods.
inventory Fundamentals
229
example Problem
A company makes and sells a seasonal product. Based on a sales forecast of 2000,
3000, 6000, and 5000 per quarter, calculate a level production plan, quarterly ending inventory, and average quarterly inventory.
If inventory carrying costs are $3 per unit per quarter, what is the annual cost of
carrying inventory? Opening and ending inventories are zero.
Answer
Quarter 1
Quarter 2
Quarter 3
Quarter 4
Total
Forecast Demand
2000
3000
6000
5000
16,000
Production
4000
4000
4000
4000
16,000
2000
3000
1000
0
Average Inventory
1000
2500
2000
500
Inventory Cost (dollars)
3000
7500
6000
1500
Ending Inventory
0
18,000
Financial statements and inventory
The two major financial statements are the balance sheet and the income statement. The
balance sheet shows assets, liabilities, and owners’ equity. The income statement shows
the revenues made and the expenses incurred in achieving that revenue.
Balance Sheet
An asset is something that has value and is expected to benefit the future operation of the
business. An asset may be tangible, such as cash, inventory, machinery, and buildings, or
may be intangible, such as accounts receivable or a patent.
Liabilities are obligations or amounts owed by a company. Accounts payable, wages
payable, and long-term debt are examples of liabilities.
Owners’ equity is the difference between assets and liabilities. After all the liabilities
are paid, it represents what is left for the owners of the business. Owners’ equity is created
either by the owners investing money in the business or through the operation of the business when it earns a profit. It is decreased when owners take money out of the business or
when the business loses money.
The accounting equation is the relationship between assets, liabilities, and owners’
equity. The equation is expressed by the balance sheet equation:
Assets = liabilities + owners’ equity
This is a basic accounting equation. Given two of the values, the third can always be
found.
example Problem
a. If the owners’ equity is $1000 and liabilities are $800, what are the assets?
b. If the assets are $1000 and liabilities are $600, what is the owners’ equity?
Answer
a. Assets = Liabilities + owners’ equity
Assets = $800 + $1000 = $1800
b. Owners’ equity = assets - liabilities
= $1000 - $600 = $400
230
Chapter nine
Balance sheet The balance sheet usually has the assets on the left side and the liabilities
and owners’ equity on the right side, as shown.
Assets
Cash
Accounts receivable
Inventory
Fixed assets
Total assets
$100,000
$300,000
$500,000
$1,000,000
Liabilities
Notes payable
Accounts payable
Long-term debt
Total liabilities
$1,900,000
Owners’ equity
Capital
Retained earnings
Total liabilities and owners’ equity
$5000
$20,000
$500,000
$525,000
$1,000,000
$375,000
$1,900,000
Capital is the amount of money the owners have invested in the company.
Retained earnings are increased by the revenues a company makes and decreased
by the expenses incurred. The summary of revenues and expenses is shown on the income
statement.
Income Statement
Income (profit) The primary purpose of a business is to increase the owners’ equity by
making a profit. For this reason, owners’ equity is broken down into a series of accounts,
called revenue accounts, which show what increased owners’ equity, and expense accounts,
which show what decreased owners’ equity.
Income = revenue - expenses
Revenue comes from the sale of goods or services. Payment is sometimes immediate
in the form of cash, but often is made as a promise to pay at a later date, called an account
receivable.
Expenses are the costs incurred in the process of making revenue. They are usually
categorized into the cost of goods sold and general and administrative expenses.
Cost of goods sold are costs that are incurred to make the product. They include
direct labor, direct material, and factory overhead. Factory overhead is all other factory
costs except direct labor and direct material, and is allocated to a product using some kind
of percentage, such as units produced.
General and administrative expenses include all other costs in running a business.
Examples of these are advertising, insurance, property taxes, and wages and benefits other
than direct material, direct labor, and factory overhead costs.
The following is an example of an income statement.
Revenue
$1,000,000
Cost of goods sold
Direct labor
$200,000
Direct material
400,000
Factory overhead
200,000
$800,000
Gross margin (profit)
$200,000
General and administrative expenses
$100,000
Net income (profit)
$100,000
example Problem
Given the following data, calculate the gross margin and the net income.
Revenue
= $1,500,000
Direct labor
= $ 300,000
Direct material
= $ 500,000
Factory overhead
= $ 400,000
General and administrative expenses = $ 150,000
inventory Fundamentals
231
How much would profits increase if, through better materials management, material
costs are reduced by $50,000?
Revenue
$1,500,000
Cost of goods sold
Direct labor
$300,000
Direct material
500,000
Overhead
400,000
$1,200,000
Gross margin (gross profit)
$300,000
General and administrative expenses
$150,000
Net income (profit)
$150,000
If material costs are reduced by $50,000, income increases by $50,000. Materials
management can have a direct impact on the bottom line—net income.
Cash Flow Analysis
When inventory is purchased as raw material, it is recorded as an asset. When it enters
production, it is recorded as work-in-process inventory, and as it is processed, its value
increases by the amount of direct labor applied to it and the overhead attributed to its processing. The material is said to absorb overhead. When the goods are ready for sale, they
do not become revenue until they are sold. However, the expenses incurred in producing
the goods must be paid for. This raises another financial issue: businesses must have the
cash to pay their bills. Cash is generated by sales, and the flow of cash into a business
must be sufficient to pay bills as they become due. Businesses develop financial statements showing the cash flows into and out of the business. Any shortfall of cash must be
provided for, perhaps by borrowing or in some other way. This type of analysis is called
cash flow analysis.
Return on Investment
Another useful measurement often used to evaluate the financial efficiency of an investment is the return on investment, or ROI. There are several possible variations for calculating ROI, but one general approach is the following simple formula:
Return on Investment =
Net profit from investment
Investment cost
The net profit from the investment is, of course, equal to the financial gain from the
investment minus the investment cost.
Financial Inventory Performance Measures
From a financial point of view, inventory is an asset and represents money that is tied up
and cannot be used for other purposes. As discussed previously in this chapter, inventory
has a carrying cost—the costs of capital, storage, and risk. Finance wants as little inventory as possible and needs some measure of the level of inventory. Total inventory investment is one measure, but in itself does not relate to sales. Two measures that do relate to
sales are the inventory turns ratio and days of supply.
Inventory turns Ideally, a manufacturer carries no inventory. This is impractical,
since inventory is needed to support manufacturing and often to supply customers. How
much inventory is enough? There is no one answer. A convenient measure of how effectively inventories are being used is the inventory turns ratio:
Inventory turns =
annual cost of goods sold
average inventory in dollars
232
Chapter nine
The calculation of average inventory can be complicated and is a subject for cost
accounting. In this text it will be taken as a given, although a simple formula under the
assumption of relatively constant demand and replenishment is often given as:
Average Inventory = 1inventory at beginning of period + inventory at end of period2/2
In some cases, inventory turns are calculated using the inventory position on the balance
sheet as the value of the average inventory.
For example, if the annual cost of goods sold is $1 million and the average inventory
is $500,000, then
Inventory turns =
$1,000,000
= 2
$500,000
What does this mean? At the very least, it means that with $500,000 of inventory a
company is able to generate $1 million in sales. If, through better materials management,
the firm is able to increase its turnover ratio to 10, the same sales are generated with only
$100,000 of average inventory. If the annual cost of carrying inventory is 25% of the
inventory value, the reduction of $400,000 in inventory results in a cost reduction (and
profit increase) of $100,000.
Inventory turns is also a convenient method to estimate the average length of time
inventory stays within the operation. For example, if inventory turnover is 4 times per
year, then that implies there is an average of 3 months of inventory in the operation
(12 months/4 turns). This concept is somewhat similar to the concept of cycle time or
throughput time (see Chapter 6). Another useful concept relating to inventory and time is
inventory velocity, which essentially means the time from receipt of the inventory as raw
material until it is sold as finished goods.
example Problem
a. What will be the inventory turns ratio if the annual cost of goods sold is $24 million
a year and the average inventory is $6 million?
Answer
Inventory turns =
=
annual cost of goods sold
average inventory in dollars
24,000,000
= 4
6,000,000
b. What would be the reduction in inventory if inventory turns were increased to
12 times per year?
Answer
Average inventory =
=
annual cost of goods sold
inventory turns
24,000,000
12
= $2,000,000
Reduction in inventory = 6,000,000 - 2,000,000 = $4,000,000
c. If the cost of carrying inventory is 25% of the average inventory, what will the
savings be?
Answer
Reduction in inventory = $4,000,000
Savings = $4,000,000 * 0.25 = $1,000,000
Days of supply Companies need to know how much inventory they have in order to
make effective decisions at both management and operational levels. One of the easiest
inventory Fundamentals
233
tools to use and understand is days of supply. It is essentially a measure of how long in
days, at the rate of anticipated demand, it will take for the current inventory level to reach
zero or a set safety stock level. This is useful for deciding when to order and also to balance item inventory levels to have equal days of supply. On the next order cycle, items
with an equal time-value of inventory can be ordered at the same time. Days of supply is
also one of the most easily understood measures and avoids the calculations of other measures such as inventory turns. The equation to calculate the days of supply is
Days of supply =
inventory on hand
average daily usage
This measure can be fairly easily converted to weeks of inventory on hand as a useful look
at total inventory that may have more relevance to people than just looking at the total
inventory count.
With the growth of the supply chain concept, the days of supply figures for inventory
can be established for each part of the supply chain. This concept, often called inventory
profiling, can be useful to see where inventory may be amassed or where performance
problems exist. The actual inventory profile can be compared to a planned profile or even
an optimal profile to establish plans for corrective actions.
example Problem
A company has 9000 units on hand and the annual usage is 48,000 units. There are
240 working days in the year. What is the days of supply?
Answer
Average daily usage =
Days of supply =
48,000
= 200 units
240
inventory on hand
9000
=
= 45 days
average daily usage
200
In general, the ability to operate a business effectively with less inventory can have a
positive impact on the financial side of the business. For example, since inventory appears
as an asset, the business would likely see an increase in cash (also an asset), particularly if
the company uses its own cash to purchase the inventory. Cash, of course, is a much more
flexible asset to have than is inventory. In addition, if the company has its cash tied up in
inventory, there is often an opportunity cost associated, meaning that the cash tied up in
inventory cannot be used to invest in other income-generating methods. If, on the other
hand, the company does not use its own cash to acquire inventory, they will typically have
to borrow the money to buy the inventory, decreasing their profit because of the need to
pay interest charges on the borrowed money. Another inventory impact on income is the
cost to handle, store, move, and risk damage to inventory. This often is included in overhead expenses, which also reduce profit as they grow. These financial impacts are one, but
only one, of several reasons that companies all over the world have embraced the concepts
of lean management to reduce inventory, discussed in much more detail in Chapter 15.
Methods of Evaluating Inventory
There are four methods accounting uses to cost inventory: first in first out, last in first out,
average cost, and standard cost. Each has implications for the value placed on inventory. If
there is little change in the price of an item, any of the four ways will produce about the same
results. However, in rising or falling prices, there can be a pronounced difference. There is
no relationship with the actual physical movement of actual items in any of the methods.
Whatever method is used is only to account for usage and the accounting value.
First in first out (FIFO) This method assumes that the oldest (first) item in stock
is used first. In rising prices, replacement is at a higher price than the assumed cost. This
method does not reflect current prices, and replacement will be understated. The reverse is
true in a falling price market.
234
Chapter nine
Last in first out (LIFO) This method assumes the newest (last) item in stock is the
first used. In rising prices, replacement is at the current price. In a falling price market
existing inventory is overvalued. However, the company is left with an inventory that may
be grossly understated in value.
Average cost This method assumes an average of all prices paid for the article. The
problem with this method in changing prices (rising or falling) is that the cost used is not
related to the actual cost.
Standard cost This method uses a cost determined before production begins. The cost
includes direct material, direct labor, and overhead. Any difference between the standard
cost and actual cost is stated as a variance.
abc inventory control
Control of inventory is exercised by controlling individual items, which are called stockkeeping units (SKUs). In controlling inventory, four questions must be answered:
1. What is the importance of the inventory item?
2. How are they to be controlled?
3. How much should be ordered at one time?
4. When should an order be placed?
The ABC inventory classification system answers the first two questions by determining the importance of items and thus allowing different levels of control based on the
relative importance of items.
Most companies carry a large number of items in stock. To have better control at a
reasonable cost, it is helpful to classify the items according to their importance. Usually
this is based on annual dollar usage, but other criteria may be used. For example, if an
item is particularly difficult to obtain and has a long replenishment lead time, that item
may be placed in a more important classification even though the annual dollar usage may
be relatively small.
The ABC principle is based on the observation that a small number of items often
dominate the results achieved in any situation. This observation was first made by an
Italian economist, Vilfredo Pareto, and is called Pareto’s law. As applied to inventories, it
is usually found that the relationship between the percentage of items and the percentage
of annual dollar usage follows a pattern in which three groups can be defined:
Group A
Group B
Group C
About 20% of the items account for about 80% of the dollar usage.
About 30% of the items account for about 15% of the dollar usage.
About 50% of the items account for about 5% of the dollar usage.
The percentages are approximate and should not be taken as absolute. This type of
distribution can be used to help control inventory.
Steps in Making an ABC Analysis
1. Establish the item characteristics that influence the results of inventory management.
This is usually annual dollar usage but may be other criteria, such as scarcity of material, very long replenishment lead times, short effective shelf life, or quality issues.
2. Classify items into groups based on the established criteria.
3. Apply a degree of control in proportion to the importance of the group.
inventory Fundamentals
235
The factors affecting the importance of an item include annual dollar usage, unit cost,
and scarcity of material. For simplicity, only annual dollar usage is used in this text. The
procedure for classifying by annual dollar usage is as follows:
1. Determine the annual usage for each item.
2. Multiply the annual usage of each item by its cost to get its total annual dollar
usage.
3. List the items according to their annual dollar usage.
4. Calculate the cumulative annual dollar usage and the cumulative percentage of
items.
5. Examine the annual usage distribution and group the items into A, B, and C groups
based on percentage of annual usage.
example Problem
A company manufactures a line of 10 items. The usage and unit cost are shown in the
following table, along with the annual dollar usage. The latter is obtained by multiplying the unit usage by the unit cost.
a. Calculate the annual dollar usage for each item.
b. List the items according to their annual dollar usage.
c. Calculate the cumulative annual dollar usage and the cumulative percentage of
items.
d. Group items into an A, B, and C classification.
Answer
a. Calculate the annual dollar usage for each item.
Part Number
Unit Usage
Unit Cost $
Annual $ Usage
1
1100
2
2200
2
600
40
24,000
3
100
4
400
4
1300
1
1300
5
100
60
6000
6
10
25
250
7
100
2
200
8
1500
2
3000
9
200
2
400
10
500
1
500
Total
5510
$38,250
Chapter nine
b. b, c, and d.
Part
Number
Annual
$ Usage
Cumulative
$ Usage
Cumulative
% $ Usage
Cumulative
% of Items
Class
2
24,000
24,000
62.75
10
A
5
6000
30,000
78.43
20
A
8
3000
33,000
86.27
30
B
1
2200
35,200
92.03
40
B
4
1300
36,500
95.42
50
B
10
500
37,000
96.73
60
C
9
400
37,400
97.78
70
C
3
400
37,800
98.82
80
C
6
250
38,050
99.48
90
C
7
200
38,250
100.00
100
C
The percentage of value and the percentage of items are often shown as a graph, as in
Figure 9.3.
Control Based on ABC Classification
Using the ABC approach, there are two general rules to follow:
1. Have plenty of low-value items (plenty in terms of days of supply). C items represent about 50% of the items but account for only about 5% of the total inventory value.
Carrying extra stock of C items adds little to the total value of the inventory. C items are
really only important if there is a shortage of one of them, when they become extremely
important, so a supply should always be on hand. For example, order a year’s supply at
a time and carry plenty of safety stock. That way, there is only one time a year when a
stockout is even possible.
100
PERCENTAGE OF VALUE
236
80
60
40
20
A
0
Figure 9.3
0
B
20
40
60
PERCENTAGE OF ITEMS
C
80
100
ABC curve: Percentage of value versus percentage of items.
inventory Fundamentals
237
2. Use the money and control effort saved to reduce the inventory of high-value
items. A items represent about 20% of the items and account for about 80% of the
value. They are extremely important and deserve the tightest control and the most
frequent review.
Different controls used with different classifications might be the following:
A items: high priority. Tight control including complete accurate records, regular
and frequent review by management, frequent review of demand forecasts, and
close follow-up and expediting to reduce lead time.
■■ B items: medium priority. Normal controls with good records, regular attention,
and normal processing.
■■ C items: lowest priority. Simplest possible controls—make sure there are plenty.
Simple or no records, such as a two-bin system or periodic review system. Order
large quantities and carry safety stock.
■■
summary
There are benefits as well as costs to having inventory. The problem is to balance the cost
of carrying inventory with the following:
Customer service. The lower the inventory level, the higher the likelihood of a stockout and the potential cost of back orders, lost sales, and lost customers. The higher the
inventory level, the higher the level of customer service.
■■ Operating efficiency. Inventories decouple one operation from another and allow
manufacturing to operate more efficiently. They allow leveling production and avoid
the costs of changing production levels. Carrying inventory allows longer production
runs and reduces the number of setups. Finally, inventories let manufacturing purchase
in larger quantities. The ABC inventory classification system prioritizes individual
items so that inventory and costs can be better controlled.
■■ Cost of placing orders. Inventory can be reduced by ordering less each time an order
is placed. However, this increases the annual cost of ordering.
■■ Transportation and handling costs. The more often goods have to be moved and the
smaller the quantities moved, the greater the transportation and material handling costs.
■■
Inventory management is influenced by several factors:
The classification of the inventory, whether raw material, work-in-process, or finished
goods.
■■ The functions that inventory serves: anticipation, fluctuation, lot size, or transportation.
■■ Supply and demand patterns.
■■ The costs associated with carrying (or not carrying) inventory.
■■
Besides managing inventory at the aggregate level, it must also be managed at the
item level. Management needs to establish decision rules about inventory items so inventory control personnel can do their job effectively.
Key terms
A items 237
ABC inventory 234
Anticipation inventories 223
Asset 229
Average cost 234
Balance sheet 230
C items 236
Capacity-associated costs 228
Capital 230
Carrying costs 227
Cost of goods sold 230
Cycle stock 224
238
Chapter nine
Days of supply 233
Decoupling inventory 225
Distribution inventories 222
Expenses 230
Finished goods 222
First in first out (FIFO) 233
Fluctuation inventory 223
General and administrative expenses 230
Hedge inventory 224
Income 230
Inventory profiling 233
Inventory turns 231
Inventory velocity 232
Item cost 227
Landed cost 227
Last in first out (LIFO) 234
Liabilities 229
Lot-size inventories 224
Maintenance, repair, and operating
supplies (MROs) 223
Owners’ equity 229
Pareto’s law 234
Pipeline or movement inventories 224
Raw materials 222
Retained earnings 230
Return on investment 231
Revenue 230
Safety stock 223
Standard cost 234
Stockout 228
Transportation inventories 224
Work-in-process (WIP) 222
Questions
1. What are inventories? Why are they important to manufacturing companies?
2. What are the responsibilities of inventory management?
3. What is aggregate inventory management? With what is it concerned?
4. What are decision rules? Why are they necessary?
5. According to the flow of material, what are the four classifications of inventories?
6. Why is less inventory needed in a line-flow manufacturing system than in lot or batch
manufacturing?
7. What is the basic purpose of inventories? In what five areas do they provide a buffer?
8. Describe the function and purpose of the following kinds of inventories:
a. Anticipation.
b. Fluctuation.
c. Lot size.
d. Transportation.
9. Describe how inventories influence each of the following:
a. Customer service.
b. Plant operations.
10. What are the five costs associated with inventories?
11. Name and describe the categories of inventory-carrying costs.
12. Name and describe the categories of ordering costs found in a factory.
13. What are stockout costs and capacity-associated costs? What is their relationship to inventories?
14. What are the balance sheet equation and the income statement equation?
15. What is the purpose of cash flow analysis?
16. What do inventory turns and days of supply measure?
17. What is the basic premise of ABC analysis? What are the three steps in making an ABC inventory analysis?
18. What are the five steps in the procedure for classifying inventory by annual dollar usage?
19. Using the items in your own kitchen, classify them in to A, B, and C categories. Are you practicing good control based on ABC classification? How?
20. What is the difference between FIFO and LIFO?
21. During times of inflation why does the value of inventory change between FIFO and LIFO
evaluations?
inventory Fundamentals
239
problems
9.1. If the transit time is 11 days and the annual demand for an item is 10,000 units, what
is the average annual inventory in transit?
Answer.
301.4 units
9.2. A company is using a carrier to deliver goods to a major customer. The annual
demand is $2,500,000, and the average transit time is 10 days. Another carrier promises to deliver in 7 days. What is the reduction in transit inventory?
9.3. Given the following percentage costs of carrying inventory, calculate the annual carrying cost if the average inventory is $1 million. Capital costs are 10%, storage costs
are 6%, and risk costs are 9%.
Answer.
$250,000
9.4. A florist carries an average inventory of $12,000 in cut flowers. The flowers require
special storage and are highly perishable. The florist estimates capital costs at 10%,
storage costs at 25%, and risk costs at 50%. What is the annual carrying cost?
9.5. Annual purchasing salaries are $65,000, operating expenses for the purchasing
department are $25,000, and inspecting and receiving costs are $25 per order. If the
purchasing department places 9000 orders a year, what is the average cost of ordering? What is the annual cost of ordering?
Answer.
Average ordering cost = $35
Annual cost = $315,000
9.6. An importer operates a small warehouse that has the following annual costs. Wages
for purchasing are $45,000, purchasing expenses are $30,000, customs and brokerage
costs are $30 per order, the cost of financing the inventory is 8%, storage costs are
7%, and the risk costs are 10%. The average inventory is $250,000, and 6000 orders
are placed in a year. What are the annual ordering and carrying costs?
9.7. A company manufactures and sells a seasonal product. Based on the sales forecast
that follows, calculate a level production plan, quarterly ending inventories, and average quarterly inventories. Assume that the average quarterly inventory is the average
of the starting and ending inventory for the quarter. If inventory carrying costs are $3
per unit per quarter, what is the annual cost of carrying this anticipation inventory?
Opening and ending inventories are zero.
Sales
Quarter 1
Quarter 2
Quarter 3
Quarter 4
1000
2000
3000
2000
Totals
Production
Ending Inventory
Average Inventory
Inventory Cost
Answer.
Annual inventory costs = $6000
9.8. Given the following data, calculate a level production plan, quarterly ending inventory, and average quarterly inventory. If inventory carrying costs are $6 per unit per
quarter, what is the annual carrying cost? Opening and ending inventories are zero.
240
Chapter nine
Forecast Demand
Quarter 1
Quarter 2
Quarter 3
Quarter 4
5000
7000
8500
9500
Totals
Production
Ending Inventory
Average Inventory
Inventory Cost
If the company always carries 100 units of safety stock, what is the annual cost of
carrying it?
9.9. Given the following data, calculate a level production plan, quarterly ending inventory, and average quarterly inventory. If inventory carrying costs are $3 per unit per
quarter, what is the annual carrying cost? Opening and ending inventories are zero.
Forecast Demand
Quarter 1
Quarter 2
Quarter 3
Quarter 4
3000
4000
6500
6500
Totals
Production
Ending Inventory
Average Inventory
Inventory Cost
9.10. If the assets are $2,000,000 and liabilities are $1,600,000, what is the owners’
equity?
Answer.
$400,000
9.11. If the liabilities are $4,000,000 and the owners’ equity is $1,200,000, what are the
assets worth?
9.12. Given the following data, calculate the gross margin and the net income.
Revenue = $3,000,000
Direct labor = $700,000
Direct material = $900,000
Factory overhead = $700,000
General and administrative expense = $300,000
Answer.
Gross margin = $700,000
Net income = $400,000
9.13. In question 9.12, how much would profit increase if the materials costs are reduced
by $200,000?
9.14. If the annual cost of goods sold is $12,000,000 and the average inventory is
$2,250,000,
a. What is the inventory turns ratio?
b. What would be the reduction in average inventory if, through better materials
management, inventory turns were increased to 10 times per year?
inventory Fundamentals
241
c. If the cost of carrying inventory is 20% of the average inventory, what is the annual savings?
Answer.
a. 5.3
b. $1,050,000
c. $210,000
9.15. If the annual cost of goods sold is $30,000,000 and the average inventory is
$5,000,000,
a. What is the inventory turns ratio?
b. What would be the reduction in average inventory if, through better materials
management, inventory turns were increased to 10 times per year?
c. If the cost of carrying inventory is 25% of the average inventory, what is the annual savings?
9.16. A company has 600 units on hand and the annual usage is 7200 units. There are 240
working days in the year. What is the days of supply?
Answer.
20 days
9.17. Over the past year, a company has sold the following 10 items. The following table
shows the annual sales in units and the cost of each item.
a. Calculate the annual dollar usage of each item.
b. List the items according to their total annual dollar usage.
c. Calculate the cumulative annual dollar usage and the cumulative percentage of
items.
d. Group the items into A, B, and C groups based on percentage of annual dollar
usage.
Part Number
Annual Unit Usage
Unit Cost $
1
21,000
1
2
5000
40
3
1600
3
4
12,000
1
5
1000
100
6
50
50
7
800
2
8
10,000
3
9
4000
1
10
5000
1
Answer.
A items 2, 5
B items 8, 1, 4
C items 10, 3, 9, 6, 7
Annual $ Usage
242
Chapter nine
9.18. Analyze the following data to produce an ABC classification based on annual dollar
usage.
Part Number
Annual Unit Usage
Unit Cost $
1
200
10
2
17,000
4
3
60,000
6
4
15,000
15
5
1500
10
6
120
50
7
25,000
2
8
700
3
9
25,000
1
10
7500
1
Annual $ Usage
9.19. a. The Ajex Company has several of items they store in inventory. The table below
shows the annual demand, and the item cost. Develop a logical classification for
the inventory based on ABC:
ITEM
C34
B99
V94
H64
P77
Y12
R74
ITEM COST
$12
$23
$19
$41
$72
$62
$33
ANNUAL USAGE (# units)
4000
8000
5500
1200
400
1100
1440
b. One additional item, the M22, has a very low usage (300 per year) and a low item
cost ($3 per unit), but has a very long lead time and is often difficult to obtain.
How should that item be handled and why?
case study 9.1
randy Smith, inventory Control Manager
Randy Smith was very proud of his new position as inventory control manager for the
Johnson Trinket Company. His primary responsibility had been fairly clearly defined:
Maintain an inventory level in the warehouse that ensures that production will not run
out of stock, yet also maintain an inventory level that will minimize inventory holding
and control costs. Since Randy had recently had a course in materials management, he
knew an approach that should help him. He decided to make a list of inventory items in
one small section of the warehouse to see if he could develop a good plan for inventory
inventory Fundamentals
243
control. If it worked in the one small section, he could expand it to the rest of the more
than 30,000 part numbers in the warehouse.
The following is the data Randy compiled:
Part number
Part unit value in $
Quantity currently
in inventory
Average annual
usage
1234
$2.50
300
3000
1235
$0.20
550
900
1236
$15.00
400
1000
1237
$0.75
50
7900
1238
$7.60
180
2800
1239
$4.40
20
5000
1240
$1.80
200
1800
1241
$0.05
10
1200
1242
$17.20
950
2000
1243
$9.00
160
2500
1244
$3.20
430
7000
1245
$0.30
500
10000
1246
$1.10
25
7500
1247
$8.10
60
2100
1248
$5.00
390
4000
1249
$0.90
830
6500
1250
$6.00
700
3100
1251
$2.20
80
6000
1252
$1.20
480
4500
1253
$5.90
230
900
When Randy scanned the list, he noticed several things that disturbed him, and he asked
one of the experienced inventory clerks. The following list summarizes the part number
issues that concerned Randy, and the explanation from the clerk:
Part number 1236, a very expensive part with almost half-a-year’s worth of inventory.
This part is used for a product that has very cyclical demand, and the busy time of
the year is about to start.
■■ Part number 1241 is very inexpensive, yet the inventory is very small. This part has
a supplier with an erratic delivery history, and the part also has a very long lead time.
A lot of 150 has been on order for some time, and is now several days past due.
■■
244
Chapter nine
Part number 1242, like 1236, is expensive with almost half a year worth of
inventory—this part is shipped to a location on the other side of the country and
is being accumulated into a large lot to save shipping costs.
■■ Part number 1246 is not too expensive, with a low inventory. This part is produced inhouse, and has a quality tolerance that the older equipment, which was used to produce
it, had a difficult time meeting. The last batch was rejected by the quality department.
■■ Part number 1253 is moderately expensive with a large inventory compared to usage.
This part was subject to a recent quality audit, and almost 150 of the items were
rejected as a result of that audit.
■■
Once Randy understood some of the issues, suddenly he did not feel quite as confident that he had the best approach in mind to control the inventory to meet the expectations of his boss.
assignment
1. Use the information above to evaluate the current situation.
2. Given your evaluation, try to develop an integrated inventory control policy that
Randy should consider.
3. Is there other information that you would like to see that might help you to make a
more effective policy? If so, what would that information be and how would you use
it to help you?
Chapter
Ten
Order QuanTiTies
inTrOducTiOn
The objectives of inventory management are to provide the required level of customer
service and to reduce the sum of all costs involved. To achieve these objectives, two basic
questions must be answered:
1. How much should be ordered at one time?
2. When should an order be placed?
Management must establish decision rules to answer these questions so inventory
management personnel know when to order and how much. Lacking any better knowledge, decision rules are often made based on what seems reasonable. Unfortunately, such
rules do not always produce the best results.
This chapter will examine methods of answering the first question, and the next chapter will deal with the second question. First, it must be determined what is being ordered
and controlled.
Stockkeeping Unit (SKU)
Control is exercised through individual items in a particular inventory. These are called
stockkeeping units (SKUs). Two white shirts in the same inventory but of different sizes
or styles would be two different SKUs. The same shirt in two different inventories would
be two different SKUs.
Lot-Size Decision Rules
APICS Dictionary, 14th edition, defines a lot, or batch, as “a quantity produced together
and sharing the same production costs and specifications.” Following are some common
decision rules for determining what lot size to order at one time.
Lot-for-lot The lot-for-lot rule says to order exactly what is needed: no more, no less.
Another way to think of this is that it really assumes a basic lot size of one, allowing the
order quantity to change whenever requirements change. This technique requires timephased information such as that provided by a material requirements plan or a master
production schedule. Since items are ordered only when needed, this system creates no
unused lot-size inventory. Because of this, it is often the preferred method for planning
“A” items (see Chapter 9) and is also used in a just-in-time or lean environment.
Fixed order quantity
Fixed order quantity rules specify the number of units to be
ordered each time an order is placed for an individual item or SKU. The quantity is sometimes arbitrary, such as 200 units at a time, but is sometimes based on an economic order
size calculation, the size of a container, or the size of a package the material comes in. The
advantage to this type of rule is that it is easily understood. The disadvantage is that it does
not minimize the costs involved.
A variation on the fixed order quantity system is the min–max system. In this system, an order is placed when the quantity available falls below the order point (discussed
245
246
Chapter ten
in Chapter 11). The quantity ordered is the difference between the actual quantity available at the time of order and the maximum. For example, if the order point is 100 units, the
maximum is 300 units, and the quantity actually available when the order is placed is 75,
the order quantity is 225 units. If the quantity actually available is 80 units, an order for
220 units is placed.
One commonly used method of calculating the quantity to order is the economic order
quantity, which is discussed in the next section.
Period order quantity Rather than ordering a fixed quantity, inventory management
can order enough to satisfy future demand for a given period of time. The question is how
many periods should be covered? The answer is given later in this chapter in the discussion on the period order quantity system.
Costs
As shown in Chapter 9, the cost of ordering and the cost of carrying inventory both depend
on the quantity ordered. Ideally, the ordering decision rules used will minimize the sum of
these two costs. The best known system is the economic order quantity.
ecOnOmic Order QuanTiTy
Assumptions
The assumptions on which the economic order quantity (EOQ) is based are as follows:
1. Demand is relatively constant and is known.
2. The item is produced or purchased in lots or batches and not continuously.
3. Order preparation costs and inventory carrying costs are constant and known.
4. Replacement occurs all at once.
These assumptions are usually valid for finished goods whose demand is independent
and fairly uniform. However, there are many situations where the assumptions are not
valid and the EOQ concept is of no use. For instance, there is no reason to calculate the
EOQ for make-to-order items in which the customer specifies the order quantity, the shelf
life of the product is short, or the length of the run is limited by tool life or raw material
batch size. In material requirements planning, the lot-for-lot decision rule is often used,
but there are also several rules used that are variations of the economic order quantity.
Development of the EOQ Formula
UNITS IN STOCK
Under the assumptions given, the quantity of an item in inventory decreases at a uniform
rate. Suppose for a particular item that the order quantity is 200 units and the usage rate is
100 units a week. Figure 10.1 shows how inventory would behave.
200
Q = Lot
Size
100
1
2
3
TIME (Weeks)
Figure 10.1 Inventory on hand over time.
4
Order Quantities
247
The vertical lines represent stock arriving all at once as the stock on hand reaches
zero. The quantity of units in inventory then increases instantaneously by Q, the quantity
ordered. This is an accurate representation of the arrival of purchased parts or manufactured parts where all parts are received at once.
From the preceding,
order quantity
200
=
= 100 units
2
2
annual demand
100 * 52
Number of orders per year =
=
order quantity
200
Average lot size inventory =
= 26 times per year
example Problem
The annual demand for an SKU is 10,075 units, and it is ordered in quantities of
650 units. Calculate the average inventory and the number of orders placed per year.
Answer
Average cycle inventory =
order quantity
650
=
= 325 units
2
2
Number of orders per year =
10,075
annual demand
=
= 15.5
order quantity
650
Notice in the example problem that the number of orders per year is rounded neither
up nor down. It is an average figure, and the actual number of orders per year will vary
from year to year but will average to the calculated figure. In the example, 16 orders will
be placed in one year and 15 in the second.
Relevant costs The relevant costs are as follows:
■■
■■
Annual cost of placing orders.
Annual cost of carrying inventory.
As the order quantity increases, the average inventory and the annual cost of carrying
inventory increase, but the number of orders per year and the ordering cost decrease. It is
a bit like a seesaw where one cost can be reduced only at the expense of increasing the
other. The key is to find the particular order quantity in which the total cost of carrying
inventory and the cost of ordering will be a minimum.
Let
A = annual usage in units
S = ordering cost in dollars per order
i = annual carrying cost rate as a decimal or a percentage
c = unit cost in dollars
Q = order quantity in units
Then,
Annual ordering cost = number of orders * costs per order
A
=
* S
Q
Annual carrying cost = average inventory * cost of carrying 1 unit for 1year
= average inventory * unit cost * carrying cost
Q
=
* c * i
2
Total annual costs = annual ordering costs + annual carrying costs
Q
A
=
* S +
* c * i
Q
2
248
Chapter ten
example Problem
The annual demand is 10,000 units, the ordering cost is $30 per order, the carrying
cost is 20%, and the unit cost is $15. The order quantity is 600 units. Calculate:
a. Annual ordering cost.
b. Annual carrying cost.
c. Total annual cost.
Answer
A = 10,000 units
S = $30
i = 0.20
c = $15
Q = 600 units
10,000
* 30
600
a. annual ordering cost =
A
* S
Q
b. annual carrying cost =
Q
600
* c * i =
* 15 * 0.2 = $ 900
2
2
c. total annual cost
=
=■■■■■$500
= ■$1400
Ideally, the total cost will be a minimum. For any situation in which the annual
demand (A), the cost of ordering (S), and the cost of carrying inventory (i) are given, the
total cost will depend upon the order quantity (Q).
Trial-and-Error Method
Consider the following example:
A hardware supply distributor carries boxes of 3-inch bolts in stock. The annual usage
is 1000 boxes, and demand is relatively constant throughout the year. Ordering costs are
$20 per order, and the cost of carrying inventory is estimated to be 20%. The cost per unit
is $5.
Let:
A = 1000 units
S = $20 per order
c = $5 per unit
i = 20% = 0.20
Then:
A
1000
* S =
* 20
Q
Q
Q
Q
Annual carrying cost =
* c * i =
* 5 * 0.20
2
2
Total annual cost = annual ordering cost + annual carrying cost
Annual ordering cost =
Figure 10.2 shows a tabulation of the costs for different order quantities. The results
from the table in Figure 10.2 are represented on the graph in Figure 10.3.
Figures 10.2 and 10.3 show the following important facts:
1. There is an order quantity in which the sum of the ordering costs and carrying costs
is a minimum.
2. This EOQ occurs when the cost of ordering equals the cost of carrying.
3. The total cost varies little for a wide range of lot sizes about EOQ.
The last point is very important for two reasons. First, it is usually difficult to
determine accurately the cost of carrying inventory and the cost of ordering. Since the
Order Quantities
Order
Quantity
(Q)
Ordering
Costs
(AS/Q)
Carrying
Costs
(Qci/ 2)
Total
Cost
50
100
150
200
250
300
350
400
$400
200
133
100
80
67
57
50
$25
50
75
100
125
150
175
200
$425
250
208
200
205
217
232
250
249
Figure 10.2 Costs for different lot sizes.
500
Total Cost
Ordering Cost
Carrying Cost
COST IN DOLLARS
400
300
200
100
0
50
100
150
200
250
300
350
400
LOT SIZE
Figure 10.3 Cost versus lot size.
total cost is relatively flat around the EOQ, it is not critical to have exact values. Good
approximations are sufficient. Second, parts are often ordered in convenient packages
such as pallet loads, cases, or dozens, and it is adequate to pick the package quantity
closest to the EOQ.
Economic Order Quantity Formula
The previous section showed that the EOQ occurred at an order quantity in which the
ordering costs equal the carrying costs. If these two costs are equal, the following formula
can be derived:
Carrying costs = ordering costs
Qic
AS
=
2
Q
Solving for Q gives
2 AS
ic
2 AS
Q =
A ic
Q2 =
250
Chapter ten
This value for the order quantity is the economic order quantity. Using the formula to
calculate the EOQ in the preceding example yields:
EOQ =
2 AS
2 * 1000 * 20
=
= 200 units
A ic
A
0.20 * 5
How to Reduce Lot Size
The EOQ formula has four variables. The EOQ will increase as the annual demand (A)
and the cost of ordering (S) increase, and it will decrease as the cost of carrying inventory
(i) and the unit cost (c) increase.
The annual demand (A) is a condition of the marketplace and is beyond the control of
manufacturing. The cost of carrying inventory (i) is determined by the product itself and
the cost of money to the company. As such, it is beyond the control of manufacturing.
The unit cost (c) is either the purchase cost of the SKU or the cost of manufacturing
the item. Ideally, both costs should be as low as possible. In any event, as the unit cost
decreases, the EOQ increases.
The cost of ordering (S) is either the cost of placing a purchase order or the cost of
placing a manufacturing order. The cost of placing a manufacturing order is made up from
production control costs and setup costs. Anything that can be done to reduce these costs
reduces the EOQ.
Lean production emphasizes reduction of setup time. There are several reasons why
this is desirable, and the reduction of order quantities is one. Chapter 15 discusses these
issues further.
VariaTiOns Of The eOQ mOdel
There are several modifications that can be made to the basic EOQ model to fit particular
circumstances. Two that are often used are the monetary unit lot-size model and the noninstantaneous receipt model.
Monetary Unit Lot Size
The EOQ can be calculated in monetary units rather than physical units. The same EOQ
formula given in the preceding section can be used, but the annual usage changes from
units to dollars.
AD = annual usage in dollars
S = ordering costs in dollars
i = carrying cost rate as a decimal or a percent
Because the annual usage is expressed in dollars, the unit cost is not needed in the
modified EOQ equation.
The EOQ in dollars is:
EOQ =
example Problem
C
2 ADS
i
An item has an annual demand of $5000, preparation costs of $20 per order, and a
carrying cost of 20%. What is the EOQ in dollars?
AD = $5000
S = $20
i = 20% = 0.20
EOQ =
C
2 ADS
2 * $5000 * $20
=
= $1000
C
0.2
i
Order Quantities
251
Noninstantaneous Receipt Model
In some cases, when a replenishment order is made, the order is not all received at one
time. The most common reason for this is that the ordered material is being produced
over an extended period of time, yet material is received for the order as it is being produced. In this case, the EOQ is modified to reflect the rate of production as related to rate
of demand:
EOQ =
where
p
2 AS
a
b
C ic p - d
p = the rate of production (in units per day) and
d = the rate of demand (in units per day)
The units of time for p and d (shown above as units per day) can be any unit of time as
long as that unit of time is the same for both.
example Problem
An item has a setup cost for production of $500 per order, and the inventory carrying
costs for the item is $12 per year. The demand for the item is constant at 11 units
per day. The production rate is 50 units per day while the item is being produced.
What is the noninstantaneous economic order quantity?
Annual demand 1A2 = 111 units per day2 * 1365 days per year2
= 4015 units per year
EOQ =
2140152 500
C
12
a
50
b
50 - 11
= 2334,583.311.32
= 655 units
QuanTiTy discOunTs
When material is purchased, suppliers often give a discount on orders over a certain size.
This can be done because larger orders reduce the supplier’s costs; to get larger orders,
suppliers are willing to offer volume discounts. The buyer must decide whether to accept
the discount, and in doing so must consider the relevant costs:
Purchase cost.
Ordering costs.
■■ Carrying costs.
■■
■■
example Problem
An item has an annual demand of 25,000 units, a unit cost of $10, an order preparation cost of $10, and a carrying cost of 20%. It is ordered on the basis of an EOQ, but
the supplier has offered a discount of 2% on orders of $10,000 or more. Should the
offer be accepted?
Answer
AD = 25,000 * $10 = $250,000
S = $10
i = 20%
EOQ =
C
2 * 250,000 * 10
= $5000
0.2
Discounted order quantity = $10,000 * 0.98 = $9800
252
Chapter ten
No discount
Discount lot size
Unit Price
$10
$9.80
Lot Size
$5000
$9800
Average Lot-Size Inventory (Qc ÷ 2)
$2500
$4900
50
25
$250,000
$245,000
Number of Orders per Year
Purchase Cost
Inventory Carrying Cost (20%)
500
980
Order Preparation Cost ($10 each)
500
250
$251,000
$246,230
Total Cost
From the preceding example problem, it can be seen that taking the discount results
in the following:
There is a saving in purchase cost.
Ordering costs are reduced because fewer orders are placed since larger quantities are
being ordered.
■■ Inventory carrying costs rise because of the larger order quantity.
■■
■■
The buyer must weigh the first two against the last and decide what to do. What
counts is the total cost. Depending on the figures, it may or may not be best to take the
discount. When considering a quantity discount, the total costs of using the discount are
usually compared with the total costs of using the EOQ method.
Order QuanTiTies fOr families Of PrOducT
When cOsTs are nOT KnOWn
The EOQ formula depends upon the cost of ordering and the cost of carrying inventory. In
practice, these costs are not necessarily known or easy to determine. However, the formula
can still be used to an advantage when applied to a family of items.
For a family of items, the ordering costs and the carrying costs are generally the same
for each item. For instance, if ordering hardware items—nuts, bolts, screws, nails, and so
on—the carrying costs would be virtually the same (storage, capital, and risk costs) and
the cost of placing an order with the supplier would be the same for each item. In cases
such as this, the cost of placing an order (S) is the same for all items in the family as is cost
of carrying inventory (i).
Now
Q =
2 ADS
C i
where A (annual demand) is in dollars.
Since S is the same for all the items and i is the same for all items, the ratio 2S , i
must be the same for all items in the family. For convenience, let
K =
2S
A i
Then,
Q = K1AD
Also,
Q =
Therefore,
AD
annual demand
=
orders per year
N
AD
N
1AD
K =
N
K1AD =
Order Quantities
253
Example Problem
Suppose there were a family of items for which the decision rule was to order each
item four times a year. Since the cost of ordering (S) and the cost of carrying inventory
(i) are not known, ordering four times a year is not based on an EOQ. Can we come up
with a better decision rule even if the EOQ cannot be calculated?
Item
Annual
Usage
Orders
per Year
Present
Lot Size
K =
1AD
1AD
N
A
$10,000
4
$2500
$100
25
B
400
4
100
20
5
C
144
4
36
12
3
12
$2636
$132
33
$1318
Average inventory =
The sum of all the lots is $2636. Since the average inventory is equal to half the
order quantity, the average inventory is $2636 , 2 = $1318.
Since this is a family of items where the preparation costs are the same and the
carrying costs are the same, the values for K = 12S , i2 1/2 should be the same for
all items. The preceding calculations show that they are not. The correct value for K is
not known, but a better value would be the average of all the values:
K =
=
a 2AD
aN
132
12
= 11
This value of K can be used to recalculate the order quantities for each item.
Item
Annual
Usage
Present
Orders
per Year
Present Lot
Size
A
$10,000
4
$2500
1AD
$100
New Lot
Size ∙
K 1AD
$1100
New Orders
per Year
N ∙ AD /Q
9.09
B
400
4
100
20
220
1.82
C
144
4
36
12
132
1.09
$10,544
12
$2636
$132
$1452
12.00
Average inventory
Item A:
$1318
$726
New lot size = K 1AD = 11 * 100 = $1100
New orders per year =
AD
10,000
=
= 9.09
Q
1100
The average inventory has been reduced from $1318 to $726 while the number of
orders per year (12) remains the same. Thus, the total costs associated with inventory
have been reduced.
PeriOd Order QuanTiTy
The economic order quantity attempts to minimize the total cost of ordering and carrying
inventory and is based on the assumption that demand is uniform. Often demand is not
uniform, particularly in material requirements planning (MRP), and using the EOQ does
not produce a minimum cost. The order quantity will often exceed the demand expected
for the next few periods, which may result in inventory carried over periods of no demand
254
Chapter ten
to avoid the cost of placing another order. However, the quantity ordered may not be
exactly enough to cover the demand in the next few periods resulting in the placement
of another order even while a remnant of the last order is available. Keeping inventory
to avoid placing orders is a good idea but not keeping enough inventory to avoid placing
orders defeats the goal of EOQ. A change in the application is required and this is demonstrated in the period order quantity method.
The period order quantity (POQ) lot-size rule is based on the same theory as the
economic order quantity. It uses the EOQ formula to calculate an economic time between
orders. Note that POQ does not calculate a quantity but actually calculates the number of
periods that are to be covered. The POQ is calculated by dividing the EOQ by the demand
rate. This produces a time interval for which orders are placed. Instead of ordering the
same quantity (EOQ), orders are placed to satisfy requirements for the calculated time
interval. The number of orders placed in a year is approximately the same as for an economic order quantity, but the amount ordered each time varies. Thus, the ordering cost is
the same as it would be using the EOQ but because the order quantities are determined by
actual demand, the carrying cost is reduced.
Period order quantity =
EOQ
average weekly usage
Note that although POQ calculates an order interval say of 2 weeks, this does
not mean that there is an order placed every two weeks. It actually means that when
an order needs to be placed, then enough inventory is ordered to cover the next two
weeks. This is especially important in discontinuous demand, as shown in the following example.
example Problem
The EOQ for an item is 2800 units, and the annual usage is 52,000 units. What is
the period order quantity?
Answer
Average weekly usage = 52,000 , 52 = 1000 per week
Period order quantity =
EOQ
average weekly usage
=
2800
= 2.8 weeks
1000
3 weeks
When an order is placed, it will cover the requirements for the next three weeks.
Notice the calculation is approximate. Precision is not important.
example Problem
Given the following MRP record and an EOQ of 250 units, calculate the planned order
receipts using the economic order quantity. Next, calculate the period order quantities and the planned order receipts. In both cases, calculate the ending inventory and
the total inventory carried over the 10 weeks.
Week
1
2
3
Net
Requirements
100
50
150
Planned Order
Receipt
4
5
6
7
8
9
10
Total
75
200
55
80
150
30
890
Order Quantities
255
Answer
EOQ = 250 units
Week
1
2
3
4
Net
Requirements
100
50
150
Planned Order
Receipt
250
Ending
Inventory
150
5
6
7
8
9
10
Total
75
200
55
80
150
30
890
250
100
200
250
200
125
175
250
120
40
140
110
1360
Period order quantity:
Weekly average demand = 890 , 10 = 89 units
POQ = 250 , 89 = 2.81 S 3 weeks
Week
1
2
3
4
Net
Requirements
100
50
150
Planned Order
Receipt
300
Ending
Inventory
200
5
6
7
8
9
10
Total
75
200
55
80
150
30
890
30
0
870
330
150
0
0
255
260
55
0
180
Notice in the example problem the total inventory is reduced from 1360 to 870 units
over the 10-week period. Note also that an order is not placed in week 4 since there is no
demand in period 4 and that the period before each order has zero inventory having used
up the previous order entirely.
Practical Considerations When Using the EOQ
Lumpy demand The EOQ assumes that demand is uniform and replenishment occurs
all at once. When this is not true, the EOQ will not produce the best results. It is better to
use the period order quantity.
Anticipation inventory Demand is not uniform, and stock must be built ahead of
periods of high demand. It is better to plan a buildup of inventory based on capacity and
future demand.
Minimum order Some suppliers require a minimum order. This minimum may be
based on the total order rather than on individual items. Often these are C items where the
rule is to order plenty, not an EOQ.
Transportation inventory As will be discussed in Chapter 13, transportation carriers
give rates based on the amount shipped. A full load costs less per ton to ship than a partial
load. This is similar to the price break given by suppliers for large quantities. The same
type of analysis can be used.
256
Chapter ten
Multiples Sometimes order size is constrained by package size. For example, a supplier may ship only in skid-load lots. In these cases, the unit used should be multiples of
the minimum package size.
Order quantities and lean production As will be discussed in Chapter 15, lean
production has a profound effect on the amount of inventory to be produced at one time.
The replenishment quantity of an item is adjusted to match the demand of the next operation in the supply chain. This adjustment leads to smaller lot sizes and is often determined
by the frequency of shipments to a customer or the size of an easily moved container
rather than by calculation.
summary
The economic order quantity is based on the assumption that demand is relatively uniform. This is appropriate for some inventories, and the EOQ formula can be used with
reasonable results. One problem in using the EOQ formula is in determining the cost of
ordering and the cost of carrying inventory. Since the total cost curve is flat at the bottom,
good guesses very often will produce an order quantity that is economical. It has also been
demonstrated that the EOQ concept can be used effectively with groups of items when the
costs of carrying and ordering are not known.
The two costs influenced by the order quantity are the cost of ordering and the cost of
carrying inventory. All methods of calculating order quantities attempt to minimize the sum
of these two costs. The period order quantity does this. It has the advantage over the EOQ
in that it is better for lumpy demand because it looks forward to see what is actually needed.
Key Terms
Economic order quantity (EOQ) 246
Fixed order quantity 245
Lot-for-lot 245
Min–max system 245
Noninstantaneous receipt model 251
Period order quantity 246
Quantity discounts 251
Stockkeeping units (SKUs) 245
QuesTiOns
1. What are the two basic questions in inventory management that are discussed in the text?
2. What are decision rules? What is their purpose?
3. What is an SKU?
4. What is the lot-for-lot decision rule? What is its advantage? Where would it be used?
5. What are the four assumptions on which economic order quantities are based? For what kind of
items are these assumptions valid? When are they not?
6. Under the assumptions on which EOQs are based, what are the formulas for average lot size
and the number of orders per year?
7. What are the relevant costs associated with the two formulas? As the order quantities increase,
what happens to each cost? What is the objective in establishing a fixed order quantity?
8. Define each of the following in your own words and as a formula:
a. Annual ordering cost.
b. Annual carrying cost.
c. Total annual cost.
9. What is the economic order quantity (EOQ) formula? Define each term and give the units used.
How do the units change when monetary units are used?
10. What are the relevant costs to be considered when deciding whether to take a quantity discount? On what basis should the decision be made?
11. What is the period order quantity? How is it established? When can it be used?
Order Quantities
257
12. How do each of the following influence inventory lot-size decisions?
a. Lumpy demand.
b. Minimum orders.
c. Transportation costs.
d. Multiples.
13. A company working toward lean will have smaller lot sizes when compared to using traditional
methods. Discuss how this will affect the costs associated with inventory. What are the controllable and the uncontrollable costs?
PrOblems
10.1. An SKU costing $10 is ordered in quantities of 500 units, annual demand is 5200
units, carrying costs are 20%, and the cost of placing an order is $50. Calculate the
following:
a. Average inventory.
b. Number of orders placed per year.
c. Annual inventory carrying cost.
d. Annual ordering cost.
e. Annual total cost.
Answer.
a. 250 units
b. 10.4 orders per year
c. Inventory carrying cost = $500
d. Annual ordering cost = $520
e. Annual total cost = $1020
10.2. If the order quantity is increased to 1000 units, recalculate problems 10.1a to 10.1e
and compare the results.
10.3. A company decides to establish an EOQ for an item. The annual demand is 400,000
units, each costing $9, ordering costs are $35 per order, and inventory carrying costs
are 22%. Calculate the following:
a. The EOQ in units.
b. Number of orders per year.
c. Cost of ordering, cost of carrying inventory, and total cost.
a. EOQ = 3761 units
b. Number of orders per year = 106
c. Annual cost of ordering = $3723
Annual cost of carrying = $3723
Annual total cost = $7446
10.4. A company wishes to establish an EOQ for an item for which the annual demand
is $800,000, the ordering cost is $32, and the cost of carrying inventory is 20%.
Calculate the following:
a. The EOQ in dollars.
b. Number of orders per year.
c. Cost of ordering, cost of carrying inventory, and total cost.
d. How do the costs of carrying inventory compare with the costs of ordering?
Answer.
Answer.
a. EOQ = $16,000
b. Number of orders per year = 50
c. Annual cost of ordering = $1600
Annual cost of carrying = $1600
Annual total cost = $3200
d. The costs of carrying equal the costs of ordering since the order
quantity used was the EOQ.
258
Chapter ten
10.5. An SKU has an annual demand of 10,000 units, each costing $15, ordering costs are
$80 per order, and the cost of carrying inventory is 25%. Calculate the EOQ in units
and then convert to dollars.
10.6. A company is presently ordering on the basis of an EOQ. The demand is 10,000
units a year, unit cost is $10, ordering cost is $30, and the cost of carrying inventory
is 20%. The supplier offers a discount of 3% on orders of 1000 units or more. What
will be the saving (loss) of accepting the discount?
Answer.
Savings = $2825.45
10.7. Refer to problem 10.3. The supplier offers a 2% discount on orders of 5000 units.
Calculate the purchase cost, the cost of ordering, the cost of carrying, and the total
cost if orders of 5000 are placed. Compare the results and calculate the savings if the
discount is taken.
10.8. The local fire department uses 10,000 alkaline flashlight batteries per year, which
cost $4 each. The cost of ordering batteries is estimated to be $50. The current interest rate suggested by the city council is 25%. The sales rep has recently suggested
that you could get a discount of 2% for orders of 2000 batteries at a time. Should you
take advantage of this special offer?
10.9. Calculate the new lot size for the following if K = 5
Item
Annual Demand
1
2500
2
900
3
121
Answer.
Item 1
Item 2
Item 3
AD
New Lot Size
AD
250
150
55
10.10. Calculate K for the following data:
Item
Annual Demand
Orders per Year
11
$14,400
5
2
4900
5
3
1600
5
Total
Answer.
K = 15.33
10.11. A company manufactures three sizes of lightning rods. Ordering costs and carrying
costs are not known, but it is known that they are the same for each size. Each size
is produced six times per year. If the demand for each size is as follows, calculate
order quantities to minimize inventories and maintain the same total number of
Order Quantities
259
runs. Calculate the old and new average inventories. Is there any change in the
number of orders per year?
Item
Annual
Usage
Present
Orders
per Year
1
$22,500
6
2
$7225
6
3
$1600
6
Present
Lot Size
AD
New Lot Size
= K AD
New Orders
per Year
N = AD /Q
Total
Average Inventory
Answer.
Average inventory with present lot sizes = $2610.40
Average inventory with new lot sizes = $2100.70
10.12. A company manufactures five sizes of screwdrivers. Ordering costs and carrying
costs are not known, but it is known that they are the same for each size. At present,
each size is produced four times per year. If the demand for each size is as follows,
calculate order quantities to minimize inventories and maintain the same total number of runs. Calculate the old and new average inventories. Is there any change in
the number of orders per year?
Item
Annual
Usage
1
$12,100
2
$8100
3
$3600
4
$1600
5
$225
Present
Orders
per Year
Present
Lot Size
AD
New Lot Size
= K AD
New Orders
per Year
N = AD/Q
Total
Average Inventory
10.13. The EOQ for an item is 800 units, and the annual usage is 2300 units. What is the
period order quantity?
Answer.
POQ = 18 weeks
260
Chapter ten
10.14. Given the following net requirements, calculate the planned order receipts based
on the period order quantity. The EOQ is 250 units, and the annual demand is
4200 units.
Week
1
2
3
4
5
6
7
8
Net Requirements
100
85
90
0
85
80
90
100
630
Planned Order Receipts
Answer.
POQ = 3 weeks
Planned order period 1 = 275 units
Planned order period 5 = 255 units
Planned order period 8 = 100 units
10.15. Given the following MRP record and an EOQ of 200 units, calculate the planned
order receipts using the economic order quantity. Next, calculate the period order
quantities and the planned order receipts. In both cases, calculate the ending inventory and the total inventory carried over the 10 weeks.
Week
1
2
3
4
5
6
7
8
9
10
Net
Requirements
75
70
60
0
100
80
70
65
0
80
Total
Planned Order
Receipt
Ending
Inventory
Answer.
POQ = 3 weeks
EOQ total ending inventory = 985 units
POQ total ending inventory = 570 units
10.16. An item has a weekly demand of 240 units throughout the year. The item has a unit
value of $42 and the company uses 20% of the item value for the annual inventory
cost. When ordered, the setup cost to produce an order is $600, and the production
process is able to produce 500 per week and deliver them weekly as produced.
What is the economic order quantity?
Chapter
eleven
Independent demand
OrderIng SyStemS
IntrOductIOn
The concept of an economic order quantity, covered in Chapter 10, addresses the question of
how much to order at one time. Another important question is when to place a replacement
order. If stock is not reordered soon enough, there will be a stockout and a potential loss in
customer service. However, stock ordered earlier than needed will create extra inventory. The
problem is how to balance the costs of carrying extra inventory against the costs of a stockout.
No matter what the items are, some rules for reordering are needed and can be as simple as order when needed, order every month, or order when stock falls to a predetermined
level. Rules are used in all parts of daily life, and they vary depending on the significance
of the item. A homemaker uses some intuitive rules to make up the weekly shopping list.
Order enough meat for a week, order salt when the box is empty, order vanilla extract if it
will be needed over the next week, and so on.
In industry there are many inventories that involve a large investment and have high
stockout costs. Controlling these inventories requires effective reorder systems. Three
basic systems are used to determine when to order:
Order point system.
Periodic review system.
■■ Material requirements planning.
■■
■■
The first two are for independent demand items; the last is for dependent demand items.
Order pOInt SyStem
When the quantity of an item on hand in inventory falls to a predetermined level, called an
order point, an order is placed. The quantity ordered is usually precalculated and based
on economic order quantity concepts.
Using this system, an order must be placed when there is enough stock on hand to
satisfy demand from the time the order is placed until the new stock arrives (called the
lead time). Suppose that for a particular item the average demand is 100 units a week and
the lead time is four weeks. If an order is placed when there are 400 units on hand, on the
average there will be enough stock on hand to last until the new stock arrives. However,
demand during any one lead-time period probably varies from the average—sometimes
more and sometimes less than the 400. Statistically, half the time the demand is greater
than average, and there is a stockout; half the time the demand is less than average, and
there is extra stock. If it is necessary to provide some protection against a stockout, safety
stock can be added. The item is ordered when the quantity on hand falls to a level equal to
the demand during the lead time plus the safety stock:
OP = DDLT + SS
where
OP = order point
DDLT = demand during the lead time
SS = safety stock
261
Chapter eleven
UNITS IN STOCK
262
Q = Order
Quantity
Order Point
Safety Stock
Lead Time
FigurE 11.1 Quantity on hand versus time: independent demand item.
It is important to note that it is the demand (and the variation in demand) during the
lead time that is important. The only time a stockout is possible is during the lead time. If
demand during the lead time is greater than expected, there will be a stockout unless sufficient safety stock is carried.
Example Problem
Demand is 200 units a week, the lead time is 3 weeks, and safety stock is 300 units.
Calculate the order point.
Answer
OP = DDLT + SS
= 200 * 3 + 300
= 900 units
Figure 11.1 shows the relationship between safety stock, lead time, order quantity,
and order point. With the order point system,
Order quantities are usually fixed.
■■ The order point is determined by the average demand during the lead time. If the average demand or the lead time changes and there is no corresponding change in the order
point, effectively there has been a change in safety stock.
■■ The intervals between replenishment are not constant but vary depending on the actual
demand during the reorder cycle.
■■ The average inventory for a period is equal to the opening inventory plus the ending
inventory, divided by 2.
■■
Period opening inventory = order quantity plus safety stock
Period ending inventory = safety stock
order quantity + safety stock + safety stock
2
order quantity
=
+ safety stock
2
Q
=
+ SS
2
Average inventory =
Example Problem
Order quantity is 1000 units and safety stock (SS) is 300 units. What is the average
inventory?
Answer
Q
+ SS
2
1000
+ 300
=
2
= 800 units
Average inventory =
independent Demand Ordering Systems
263
Determining the order point depends on the demand during the lead time and the
safety stock required.
Methods of estimating the demand during the lead time were discussed in Chapter 8.
In this chapter, we discuss the factors to consider when determining safety stock.
determInIng Safety StOck
Safety stock is intended to protect against uncertainty in supply and demand. Uncertainty
may occur in two ways: quantity uncertainty and timing uncertainty. Quantity uncertainty occurs when the amount of supply or demand varies; for example, if the demand is
greater or less than expected in a given period. Timing uncertainty occurs when the time
of receipt of supply or demand differs from that expected. A customer or a supplier may
change a delivery date, for instance.
There are two ways to protect against uncertainty: carry extra stock, called safety
stock, or order early, called safety lead time. Safety stock is a calculated extra amount
of stock carried and is generally used to protect against quantity uncertainty. Safety lead
time is used to protect against uncertainty in delivery lead time by planning order releases
and order receipts earlier than required. Both safety stock and safety lead time result in
extra inventory, but the methods of calculation are different.
Safety stock is the most common way of buffering against uncertainty and is the
method described in this text. The safety stock required depends on the following:
Variability of demand during the lead time. The higher the variability, the greater the
need for safety stock.
■■ Frequency of reorder. If orders are placed frequently, changes to the demand or variability of the demand will be detected earlier.
■■ Service level desired. Higher service levels require more inventory, to accommodate
periods of increased demand.
■■ Length of the lead time. The longer the lead time, the more safety stock has to be carried to provide a specified service level. This is one reason it is important to reduce
lead times as much as possible.
■■
Variation in Demand During Lead Time
Chapter 8 discussed forecast error and it was determined that actual demand varies from
forecast for two reasons: bias error in forecasting the average demand and random variations
in demand about the average. It is the latter for which safety stock should be determined.
Suppose two items, A and B, have a 10-week sales history, as shown in Figure 11.2.
Average demand over the lead time of one week is 1000 per week for both items. (Note
Week
Item A
Item B
1
2
3
4
5
6
7
8
9
10
1200
1000
800
900
1400
1100
1100
700
1000
800
400
600
1600
1300
200
1100
1500
800
1400
1100
Total
Average
10,000
1000
10,000
1000
FigurE 11.2 Actual demand for two items.
Chapter eleven
that, for simplicity, the lead time and the sales history interval are both expressed as one
week. This is unusual, and the difference between order lead time and sales interval will
be accommodated later in this chapter.) However, the weekly demand for item A has a
range from 700 to 1400 units a week and for item B the range is from 200 to 1600 units
per week. The demand for B is more erratic than that for A. If the order point is 1200 units
for both items, there will be one stockout for A and four for B, which will occur when the
demand during any one week exceeds 1200 units. If the same service level is to be provided (the same chance of stockout for all items), some method of estimating the randomness of item demand is needed.
Variation in Demand About the Average
Suppose over the past 100 weeks a history of weekly demand for a particular item shows
an average demand of 1000 units. As expected, most of the demands are around 1000;
a smaller number would be farther away from the average, and still fewer would be farthest
away. If the weekly demands are classified into groups or ranges about the average, a
picture of the distribution of demand about the average appears. Suppose the demand is
distributed as follows:
Weekly Demand
725–774
775–824
825–874
875–924
925–974
975–1024
1025–1074
1075–1124
1125–1174
1175–1224
1225–1274
Number of Weeks
2
3
7
12
17
20
17
12
7
3
2
These data are plotted to give the results shown in Figure 11.3. This type of chart is a
histogram.
25
20
FREQUENCY
264
15
10
5
0
725 775 825 875 925 975 1025 1075 1125 1175 1225
WEEKLY DEMAND
FigurE 11.3 Histogram of actual demand.
independent Demand Ordering Systems
2.1%
.2%
−3
2.1%
34.1% 34.1%
13.6%
−2
.2%
13.6%
−1
0
265
+1
+2
+3
Standard Deviation
FigurE 11.4 Normal distribution.
Normal distribution Everything in life varies, even identical twins in some respects.
The pattern of demand distribution about the average will differ for different products and
markets. Some method is needed to describe the distribution—its shape, center, and spread.
The shape of the histogram in Figure 11.3 indicates that although there is variation
in the distribution, it follows a definite pattern, as shown by the smooth curve. Such a
natural pattern shows predictability. As long as the demand conditions remain the same,
the pattern can be expected to remain very much the same. If the demand is erratic, so
is the demand pattern, making it difficult to predict future demand with any accuracy.
Fortunately, most demand patterns are stable and predictable.
The most common predictable pattern is similar to the one outlined by the histogram
in Figure 11.3 and is called a normal curve, or bell curve, because its shape resembles a
bell. The shape of a perfectly normal distribution is shown in Figure 11.4.
The normal distribution has most of the values clustered near a central point with
progressively fewer results occurring away from the center. It is symmetrical about this
central point in that it spreads out evenly on both sides.
The normal curve is described by two characteristics. One relates to its central tendency, or average, and the other to the variation, or dispersion, of the actual values about
the average.
Average or mean The average or mean value is at the high point of the curve. It
is the central tendency of the distribution. The symbol for the mean is x (pronounced
“x bar”). It is calculated by adding the data and dividing by the total number of data. In
mathematical terms, it can be written as:
x =
ax
n
where x stands for the individual data [(in this case, the individual demands) a
(capital Greek letter sigma) is the summation sign and n is the number of data (demands)].
Example Problem
Given the following actual demands for a 10-week period, calculate the average (x) of
the distribution.
266
Chapter eleven
Period
Actual Demand
1
1200
2
1000
3
800
4
900
5
1400
6
1100
7
1100
8
700
9
1000
10
800
Total
10,000
Answer
Average Demand = x =
10,000
ax
=
= 1000 units
n
10
Dispersion The variation, or dispersion, of actual demands about the average refers to
how closely the individual values cluster around the mean or average. It can be measured
in several ways:
As a range of the maximum minus the minimum value.
As the mean absolute deviation (MAD), which is a measure of the average forecast
error. (Calculation of MAD was discussed in Chapter 8.)
■■ As a standard deviation.
■■
■■
Standard Deviation (𝛔)
The standard deviation is a statistical value that measures how closely the individual values cluster about the average. It is represented by the Greek letter sigma (σ). The standard
deviation is calculated as follows:
1. Calculate the deviation for each period by subtracting the actual demand from the
forecast demand.
2. Square each deviation.
3. Add the squares of the deviations.
4. Divide the value in step 3 by the number of periods to determine the average of the
squared deviations.
5. Calculate the square root of the value calculated in step 4. This is the standard deviation (σ).
6. Most calculators and spreadsheet applications have a statistical function that can calculate sigma (σ) directly and have two variations of sigma: sn, which is based on a
population and sn–1, which is based on a sample estimating the population. Sigma is
used based on n (sn). Further explanation of this difference is unfortunately beyond
the scope of this text.
independent Demand Ordering Systems
267
Note that previous versions of this text have used MAD as the measure of dispersion
since it is easier to calculate manually. However, standard deviation is a more widely
accepted technique, which will be used in the calculation of safety stocks. One standard
deviation is approximately equal to 1.25 MADs.
It is also important to note that in the discussion of safety stock the deviations in
demand are still calculated for the same time intervals as the lead time. If the lead time is
one week, then the variation in demand over a one-week period is needed to determine the
safety stock. Differences in the lead time and forecast interval will be discussed later in
this chapter.
Example Problem
Given the data from the previous example problem, calculate the standard deviation (σ).
Answer
Period
Forecast Demand
Actual Demand
Deviation
Deviation Squared
1
1000
1200
200
40,000
2
1000
1000
0
0
3
1000
800
−200
40,000
4
1000
900
−100
10,000
5
1000
1400
400
160,000
6
1000
1100
100
10,000
7
1000
1100
100
10,000
8
1000
700
−300
90,000
9
1000
1000
0
0
10
1000
800
−200
40,000
Total
10,000
10,000
0
400,000
Average of the squares of the deviation = 400,000 , 10 = 40,000
Sigma = 240,000 = 200 units
From statistics, we can determine that:
The actual demand will be within ;1 sigma (;200 units) of the forecast average approximately 68% of the time.
The actual demand will be within ;2 sigma (;400 units) of the forecast average approximately 95% of the time.
The actual demand will be within ;3 sigma (;600 units) of the forecast average approximately 99.7% of the time.
Determining the Safety Stock and Order Point
Now that the standard deviation has been calculated, how much safety stock is needed can
be determined.
One property of the normal curve is that it is symmetrical about the average. This
means that half the time the actual demand is less than the average and half the time it is
268
Chapter eleven
greater. Safety stocks are needed to cover only those periods in which the demand during
the lead time is greater than the average. Thus, a service level of 50% can be attained with
no safety stock. If a higher service level is needed, safety stock must be provided to protect against those times when the actual demand is greater than the average.
As stated, statistics have shown that the error is within ±1 sigma of the forecast about
68% of the time (34% of the time less and 34% of the time greater than the forecast).
Suppose the standard deviation of demand during the lead time is 100 units and this
amount is carried as safety stock. This much safety stock provides protection against stockout for the 34% of the time that actual demand is greater than expected. In total, there is
enough safety stock to provide protection for the 84% of the time 150% + 34% = 84%2
that a stockout is possible.
The service level is a statement of the percentage of time there is no stockout. But
what exactly is meant by supplying the customer 84% of the time? It means being able to
supply when a stockout is possible, and a stockout is possible only during the time interval
between when an order is to be placed and when the replenishment is received. If an order
is placed 100 times a year, there are 100 chances of a stockout. With safety stock equivalent to one standard deviation, on the average one would expect no stockouts about 84 of
the 100 times.
It should be noted that there are other definitions and interpretations of the concept of
service level, but a complete treatment of additional interpretations will yield fairly small
variations in the safety stock and are beyond the scope of this text.
Example Problem
Using the figures in the last example problem, in which the standard deviation was
calculated as 200 units, and assuming the lead time to be one period,
a. Calculate the safety stock and the order point for an 84% service level.
b. If a safety stock equal to two standard deviations is carried, calculate the safety
stock and the order point.
Answer
a. Safety stock = 1 sigma
= 1 * 200
= 200 units
Order point = DDLT + SS
= 1000 + 200 = 1200 units
where DDLT and SS are as defined previously. With this order point and level of safety
stock, on the average there are no stockouts 84% of the time when a stockout is
possible.
b. SS = 2 * 200
= 400 units
OP = DDLT + SS
= 1000 + 400
= 1400 units
Safety factor The service level is directly related to the number of standard deviations
provided as safety stock and is usually called the safety factor. In earlier discussions, the
normal curve has been used to determine the service level produced by one, two, or three
standard deviations. Safety factors are the inverse of this procedure, wherein one can
determine a desired service level and look up the corresponding number of standard deviations required.
Figure 11.5 shows safety factors for various service levels. Note that the service level
is the percentage of order cycles without a stockout. For values not shown on the table, a
close approximation of the safety factor can be made by interpolating the factors given.
For example, to find the safety factor for a desired service level of 77%, calculate the
independent Demand Ordering Systems
Service Level (%)
Safety Factor
50
75
80
85
90
94
95
96
97
98
99
99.86
99.99
0.00
0.67
0.84
1.04
1.28
1.56
1.65
1.75
1.88
2.05
2.33
3.00
4.00
269
FigurE 11.5 Table of safety factors.
average safety factor for a 75% service level (0.67) and an 80% service level (0.84). The
safety factor for a 77% service level would be approximately
0.67 + 0.84
= 0.76
2
Example Problem
If the standard deviation is 200 units, what safety stock should be carried to provide
a service level of 90%? If the expected demand during the lead time is 1500 units,
what is the order point?
Answer
From Figure 11.5, the safety factor for a service level of 90% is 1.28. Therefore,
Safety stock = s * safety factor
= 200 * 1.28
= 256 units
Order point = DDLT + SS
= 1500 + 256
= 1756 units
determInIng ServIce levelS
Theoretically, a company wants to carry enough safety stock on hand so the cost of carrying the extra inventory plus the cost of stockouts is a minimum. Stockouts cost money for
the following reasons:
Backorder costs.
■■ Lost sales.
■■ Lost customers.
■■
The cost of a stockout varies depending on the item, the market served, the customer,
and competition. In some markets, customer service is a major competitive tool, and a
stockout can be very expensive. In others, it may not be a major consideration. Stockout
costs are difficult to establish. Usually the decision about what the service level should
be is a senior management decision and is part of the company’s corporate and marketing
strategy. As such, it is beyond the scope of this text.
The only time it is possible for a stockout to occur is when stock is running low, and
this happens every time an order is to be placed. Therefore, the chances of a stockout are
Chapter eleven
STOCK ON HAND
270
Two Exposures
One Exposure
FigurE 11.6 Exposures to stockout.
directly proportional to the frequency of reorder. The more often stock is reordered, the
more often there is a chance of a stockout. Figure 11.6 shows the effect of the order quantity
on the number of exposures per year. Note also that when the order quantity is increased,
exposure to stockout decreases. The safety stock needed decreases, but because of the larger
order quantity, the average inventory increases. When a company pursues a lean production
approach, the tactics to reduce average inventory tends to drive the order quantity down significantly as a result of reducing order cost. While this will often increase the exposures to
stockout by a large number, other tactics are used to minimize the risks associated with those
stockouts. Lean production is discussed more completely in Chapter 15.
It is the responsibility of management to determine the number of stockouts per year
that are tolerable. Then the service level, safety stock, and order point can be calculated.
Example Problem
Suppose management stated that it could tolerate only one stockout per year for a
specific item.
For this particular item, the annual demand is 52,000 units, it is ordered in quantities of 2600, and the standard deviation of demand during the lead time is 100 units.
The lead time is one week. Calculate:
a. Number of orders per year.
b. Service level.
c. Safety stock.
d. Order point.
Answer
a. Number of orders per year =
=
annual demand
order quantity
52,000
= 20 times per year
2600
b. Since one stockout per year is tolerable, there must be no stockouts 19 (20 − 1)
times per year.
Service level =
20 - 1
= 95%
20
c. From Figure 11.5
Safety factor = 1.65
Safety stock = safety factor * s
= 1.65 * 100 = 165 units
(52,000)
= 1000 units
52
Order point = demand during lead time + SS
d. Demand during lead time 11 week2 =
= 1000 + 165 = 1165 units
independent Demand Ordering Systems
271
dIfferent fOrecaSt and lead-tIme IntervalS
Usually, there are many items in an inventory, each with different lead times. Records of
actual demand and forecasts are normally made on a weekly or monthly basis for all items
regardless of what the individual lead times are. It is almost impossible to measure the
variation in demand about the average for each of the lead times. Some method of adjusting standard deviation for the different time intervals is needed.
If the lead time is zero, the standard deviation of demand is zero. As the lead time
increases, the standard deviation increases. However, it will not increase in direct proportion to the increase in time. For example, if the standard deviation is 100 for a lead time of
one week, then for a lead time of four weeks it will not be 400, since it is very unlikely that
the deviation would be high for four weeks in a row. As the time interval increases, there
is a smoothing or canceling effect, and the longer the time interval, the more smoothing
takes place.
The following adjustment can be made to the standard deviation or the safety stock to
compensate for differences between lead time interval (LTI) and forecast interval (FI). It
states that the standard deviation changes as the square root of the change in the interval.
Although not exact, the formula gives a good approximation.
sigma for LTI = 1sigma for FI2
LTI
A FI
Example Problem
The forecast interval is 4 weeks, the lead time interval is 2 weeks, and sigma for
the forecast interval is 150 units. Calculate the standard deviation for the lead-time
interval.
Answer
sigma for LTI = 150 224 = 150 * 0.707 = 106 units
The preceding relationship is also useful where there is a change in the LTI. Now it is
probably more convenient to work directly with the safety stock rather than the standard
deviation. The relationship is as follows:
New safety stock = old safety stock
new interval
A old interval
Example Problem
The safety stock for an item is 150 units, and the lead time is 2 weeks. If the lead
time changes to 3 weeks, calculate the new safety stock.
Answer
SS 1new2 = 150232
= 150 * 1.22 = 183 units
It should also be noted that as the lead time is decreased, the amount of safety stock is
also decreased. This is one of the many direct benefits of just-in-time and lean production
discussed in Chapter 15.
determInIng When the Order pOInt IS reached
There must be some method to show when the quantity of an item on hand has reached
the order point. In practice, there are many systems, but they all are inclined to be variations or extensions of three basic systems: the two-bin system, the kanban system, and the
perpetual inventory record system.
272
Chapter eleven
Two-Bin System
A quantity of an item equal to the order point quantity is set aside, frequently in a separate
or second bin, and not touched until all the main stock is used up. When this stock needs to
be used, the production control or purchasing department is notified and a replenishment
order is placed.
There are variations on this system, such as the red tag system, where a tag is placed
in the stock at a point equal to the order point. Bookstores frequently use this system. A
tag or card is placed in a book that is in a stack in a position equivalent to the order point.
When a customer takes that book to the checkout, the store is effectively notified that it is
time to reorder that title.
The two-bin system is a simple way of keeping control of C items. Because they
are of low value, it is best to spend the minimum amount of time and money controlling
them. However, they do need to be managed, and someone should be assigned to ensure
that when the reserve stock is reached an order must be placed. When it is out of stock, a
C item becomes an A item.
Kanbans
The kanban system is a simple system that signals the need for more product. It normally
consists of a card or ticket that has information on the item and the quantity to be produced. It avoids the need for formal record keeping and, like the two-bin system, makes a
visual signal of the need for more product when the inventory falls below a preset level.
It is used to replenish all items and not just low-value C items. Kanbans are discussed in
detail in Chapter 15.
Perpetual Inventory Record System
A perpetual inventory record is a continual account of inventory transactions as they
occur. At any instant, it holds an up-to-date record of transactions. At a minimum, it contains the balance on hand, but it may also contain the quantity on order but not received,
the quantity allocated but not issued, and the available balance. The accuracy of the record
depends upon the speed with which transactions are recorded and the accuracy of the
input. Because manual systems rely on the input of humans, they are more likely to have
slow response and inaccuracies. Computer-based systems have a higher transaction speed
and reduce the possibility of human error.
An inventory record contains variable and permanent information. Figure 11.7 shows
an example of a perpetual inventory record.
Permanent or static information is shown at the top of Figure 11.7. Although not
absolutely permanent, this information does not change frequently. Any alteration is
426254 SCREW
DATE
01
02
03
04
05
ORDERED
ORDER
QUANTITY
500
RECEIVED
ISSUED
500
400
500
FigurE 11.7 Perpetual inventory record.
ON HAND
500
500
500
100
600
ORDER
POINT
100
ALLOCATED
400
0
0
AVAILABLE
500
100
100
100
600
independent Demand Ordering Systems
273
usually the result of an engineering change, manufacturing process change, or inventory
management change. It includes data such as the following:
Part number, name, and description.
Storage location.
■■ Order point.
■■ Order quantity.
■■ Lead time.
■■ Safety stock.
■■ Suppliers.
■■
■■
Variable or dynamic information changes with each transaction and includes the
following:
Quantities ordered: dates, order numbers, and quantities.
Quantities received: dates, order numbers, and quantities.
■■ Quantities issued: dates, order numbers, and quantities.
■■ Balance on hand.
■■ Allocated: dates, order numbers, and quantities.
■■ Available balance.
■■
■■
The information depends on the needs of the company and the particular
circumstances.
perIOdIc revIeW SyStem
In the order point system, an order is placed when the quantity on hand falls to a predetermined level called the order point. The quantity ordered is usually predetermined on some
basis such as the economic order quantity. The interval between orders varies depending
on the demand during any particular cycle. In most retail environments and especially
supermarkets, there are thousands of items, and each item could be reaching its order
point at any time. It is impractical to place many individual orders and it is anticipated that
many items will be delivered on one truck at a time. In this way, the ordering cost, which
includes the price of transportation, is spread over many small items. The periodic review
system makes the timing of each order a regular interval.
Using the periodic review system, the quantity on hand of a particular item is determined
at specified, fixed-time intervals, and an order is placed. Figure 11.8 illustrates this system.
Figure 11.8 shows that the review intervals (t1, t2, and t3) are equal and that Q1, Q2,
and Q3 are not necessarily the same. Thus, the review period is fixed, and the order quantity is allowed to vary. The quantity on hand plus the quantity ordered must be sufficient
UNITS IN STOCK
TARGET LEVEL
Q2
Q1
L
L
R1
Q3
L
R2
L
R3
FigurE 11.8 Periodic review system: Units in stock versus time.
274
Chapter eleven
to last until the next shipment is received. That is, the quantity on hand plus the quantity
ordered must equal the sum of the demand during the lead time plus the demand during
the review period plus the safety stock.
Target-Level or Maximum-Level Inventory
The quantity equal to the demand during the lead time plus the demand during the review
period plus safety stock is called the target-level or maximum-level inventory:
T = D1R + L2 + SS
where
T = target (maximum) inventory level
D = demand per unit of time
L = lead-time duration
R = review-period duration
SS = safety stock
Note that, as illustrated in Figure 11.8, inventory rarely actually reaches the target or
maximum level inventory since product will continue to be consumed during the lead time
interval.
The order quantity is equal to the maximum inventory level minus the quantity on
hand at the review period:
Q = T - I
where
Q = order quantity
I = inventory on hand
The periodic review system is useful for the following:
Where there are many small issues from inventory, and posting transactions to inventory records are very expensive. Supermarkets and retailers are in this category.
■■ Where ordering costs per item need to be kept small. This occurs when many different
items are ordered from one source. A regional distribution center may order most or
all of its stock from a central warehouse in regular, large, multi-item shipments.
■■ Where many items are ordered together to make up a production run or fill a truckload. A good example of this is a regional distribution center that orders a truckload
once a week from a central warehouse.
■■
Example Problem
A hardware company stocks nuts and bolts and orders them from a local supplier once
every 2 weeks (10 working days). Lead time is 2 days. The company has determined
that the average demand for ½-inch bolts is 150 per week (5 working days), and it
wants to keep a safety stock of 3 days’ supply on hand. An order is to be placed this
week, and stock on hand is 130 bolts.
a. What is the target level?
b. How many ½-inch bolts should be ordered this time?
Answer
Let
D = demand per unit of time = 150 , 5 = 30 per working day
L = lead-time duration = 2 days
R = review period duration = 10 days
SS = safety stock = 3 days’ supply = 3 * 30 = 90 units
I = inventory on hand = 130 units
independent Demand Ordering Systems
275
Then,
a. Target level T = D1R + L2 + SS
= 30110 + 22 + 90
= 450 units
b. Order quantity Q = T - I
= 450 - 130 = 320 units
dIStrIbutIOn InventOry
Distribution inventory includes all the finished goods held anywhere in the distribution
system. The purpose of holding inventory in distribution centers is to improve customer
service by locating stock near the customer and to reduce transportation costs by allowing
the manufacturer to ship full loads rather than partial loads over long distances. This will
be studied in Chapter 13.
The objectives of distribution inventory management are to provide the required level
of customer service, to minimize the costs of transportation and handling, and to be able to
interact with the factory to minimize scheduling problems.
Distribution systems vary considerably, but in general they have a central supply
facility that is supported by a factory, a number of distribution centers, and, finally, customers. Figure 11.9 shows a schematic of such a system. The customers may be the final
consumer or some intermediary in the distribution chain.
Unless a firm delivers directly from factory to customer, demand on the factory is
created by central supply. In turn, demand on central supply is created by the distribution
centers. This can have severe repercussions on the pattern of demand on central supply
and the factory. Although the demand from customers may be relatively uniform, the
demand on central supply is not, because it is dependent on when the distribution centers
place replenishment orders. In turn, the demand on the factory depends on when central
supply places orders. Figure 11.10 shows the process schematically.
Notice in Figure 11.10 how continuous, steady demand at the distribution center has
changed, due to lot sizing into discontinuous or lumpy demand at the central supply. The
distribution system is the factory’s customer, and the way the distribution system interfaces with the factory has a significant effect on the efficiency of factory operations.
Distribution inventory management systems can be classified into decentralized, centralized, and distribution requirements planning.
FACTORY
CENTRAL
SUPPLY
DISTRIBUTION
CENTER A
DISTRIBUTION
CENTER B
CUSTOMERS
FigurE 11.9 Schematic of a distribution system.
DISTRIBUTION
CENTER C
276
Chapter eleven
DEMAND
Demand at distribution
center is uniform.
0
TIME
Order
Point
DISTRIBUTION
INVENTORY
LEVELS
0
Distribution centers
order when their order
point is reached.
TIME
ORDER
ORDER
REORDER
QUANTITY
CENTRAL
SUPPLY
DEMAND
Demand at central
supply is lumpy.
0
TIME
FigurE 11.10 Distribution inventory.
Decentralized System
In a decentralized system, each distribution center first determines what it needs and
when, and then places orders to central supply. Each center orders on its own to service
local demand without regard for the needs of other centers, available inventory at central
supply, or the production schedule of the factory.
The advantage of the decentralized system is that each center can operate on its own
and thus reduce communication and coordination expense. The disadvantage is the lack
of coordination and the effect this may have on overall inventories, customer service, and
factory schedules. Because of these deficiencies, some distribution systems have moved
toward more central control.
A number of ordering systems can be used at each distribution center, including the
order point and periodic review systems. The decentralized system is sometimes called the
pull system because orders are placed on central supply and pulled through the system.
Centralized System
In a centralized system, all forecasting and order decisions are made centrally. Stock is
pushed out into the system from central supply. Distribution centers have no say about
what they receive.
Different ordering systems can be used, but generally an attempt is made to replace
the stock that has been sold and to provide for special situations such as seasonality or
sales promotions. These systems attempt to balance the available inventory with the needs
of each distribution center.
The advantage of these systems is the coordination between factory, central supply,
and distribution center needs. The disadvantage is the inability to react to local demand,
thus lowering the level of service.
Distribution Requirements Planning
Distribution requirements planning (DRP) is a system that forecasts when the various
demands will be made by the system on central supply. This gives central supply and the
factory the opportunity to plan for the goods that will actually be needed and when. It is
able both to respond to local customer demand and coordinate planning and control.
independent Demand Ordering Systems
Distribution Center A
Part 5678
277
Distribution Center B
Part 5678
Week
1
Planned Order Release
200
2
3
Week
1
200
Planned Order Release
2
3
100
Central Supply
Part 5678
Lead Time: 2 weeks
Order Quantity: 500
Week
1
2
3
Gross Requirements
200 100 200
Scheduled Receipts
Projected Available
400 200 100 400
500
Planned Order Release
FigurE 11.11 Distribution requirements planning.
The system translates the logic of material requirements planning to the distribution
system. Planned order releases from the various distribution centers become the input
to the material plan of central supply. The planned order releases from central supply
become the forecast of demand for the factory master production schedule. Figure 11.11
shows the system schematically. The records shown are all for the same part number.
Example Problem
A company making lawnmowers has a central supply attached to its factory and two
distribution centers. Distribution center A forecasts demand at 25, 30, 55, 50, and
30 units over the next 5 weeks and has 100 lawnmowers in transit that are due in
week 2. The transit time is 2 weeks, the order quantity is 100 units, and there are 50
units on hand. Distribution center B forecasts demand at 95, 85, 100, 70, and 50
over the next 5 weeks. Transit time is 1 week, the order quantity is 200 units, and
there are 100 units on hand. The central warehouse has a lead time of 2 weeks, the
order quantity is 500 units, and there are 400 on hand. Calculate the gross requirements, projected available, and planned order releases for the two distribution centers, and the gross requirements, projected available, and planned order releases for
the central warehouse.
Answer
Distribution Center A
Transit Time:
2 weeks
Order Quantity: 100 units
Week
1
2
3
4
5
Gross Requirements
25
30
55
50
30
40
90
60
In Transit
Projected Available
Planned Order Release
100
50
25
95
100
278
Chapter eleven
Distribution Center B
Transit Time:
1 week
Order Quantity: 200 units
Week
1
2
3
4
5
Gross Requirements
95
85
100
70
50
5
120
20
150
100
4
5
In Transit
Projected Available
100
Planned Order Release
200
200
Central Supply
Lead Time:
2 weeks
Order Quantity: 500 units
Week
Gross Requirements
1
2
3
200
100
200
200
100
400
Scheduled Receipts
Projected Available
400
Planned Order Release
500
Summary
This chapter addresses the problem of when to order, whereas Chapter 10 addressed the
problem of how much to order. Systems for ordering should be simple to use, such as an
order point system or a periodic review system. Ordering systems need to ensure orders
are placed in time to avoid running out of stock. Statistical applications allow a prediction
of the demand during the critical lead time to establish safety stocks. The order point uses
this safety stock plus anticipated demand during the lead time to ensure acceptable service
to the customer. Distribution systems can take advantage of MRP-type logic to optimize
order quantities all the way through the supply chain. Inventory and ordering systems are
all affected by the lead time; discussion of techniques of shortening the lead time is a subject of most of the chapters in this book.
key termS
Average or mean 265
Centralized system 276
Decentralized system 276
Dispersion 266
Distribution requirements planning
(DRP) 276
Kanban system 272
Lead time 261
independent Demand Ordering Systems
Normal curve or bell curve 265
Normal distribution 265
Order point 261
Permanent or static information 272
Perpetual inventory record 272
Safety factor 268
Safety lead time 263
279
Safety stock 263
Standard deviation 266
Target-level or maximum-level
inventory 274
Two-bin system 272
Variable or dynamic information 273
QueStIOnS
1. What are independent demand items? What two basic ordering systems are used for these
items? What are dependent demand items? What system should be used to order these items?
2. a. Using the order point system, when must an order be placed?
b. Why is safety stock carried?
c. What is the formula for the order point?
d. On what two things does the order point depend?
e. Why is the demand during the lead time important?
3. What are four characteristics of the order point system?
4. What are the four factors that can influence the amount of safety stock that should be carried?
How does the length of the lead time affect the safety stock carried?
5. What is a normal distribution? What two characteristics define it? Why is it important in determining safety stock?
6. What is the standard deviation of demand during the lead time? If the standard deviation for the
lead-time interval is 100 units, what percentage of the time would the actual demand be equal
to {100 units ? To {200 units ? To {300 units ?
7. What is service level?
8. What are the three categories of stockout costs? What do these costs depend upon in any
company?
9. Why does the service level depend upon the number of orders per year?
10. If the lead time increases from 1 to 4 weeks, will the standard deviation of demand during the
lead time increase four times? If not, why not?
11. Describe the two-bin system.
12. What kinds of information are shown on a perpetual inventory record?
13. What are the differences between the order point system and the periodic review system
regarding when orders are placed and the quantity ordered at any one time?
14. Define the target level used in the periodic review system.
15. Describe how changes in demand will affect the order quantity when using the period review
system.
16. What are the objectives of distribution inventory management?
17. If a factory does not supply the customer directly, from where does demand on the factory
come? Is it independent or dependent demand?
18. Describe and compare the pull and push systems of inventory management.
19. Describe distribution requirements planning.
20. Grocery stores are an example of well-controlled inventory and replenishment systems.
Describe in your own words examples of safety stock, the two-bin system, and the periodic
review system, including target level, review period, replenishment period, and order quantity.
21. Describe the advantages and disadvantages of a decentralized distribution system.
22. Describe how the pattern of continuous demand changes from a distribution center to central
supply.
280
Chapter eleven
prOblemS
11.1. For a particular SKU, the lead time is 4 weeks, the average demand is 150 units per
week, and safety stock is 100 units. What is the average inventory if 1600 units are
ordered at one time? What is the order point?
Answer.
Average inventory = 900 units
Order point = 700 units
11.2. For a particular SKU, the lead time is 6 weeks, the average demand is 90 units a
week, and safety stock is 200 units. What is the average inventory if 10 weeks’ supply is ordered at one time? What is the order point?
11.3. Given the following data, calculate the average x of the distribution and the standard
deviation (σ).
Period
Actual
Demand
1
500
2
600
3
425
4
450
5
600
6
575
7
375
8
475
9
525
10
475
Deviation
Total
Answer.
Average demand x = 500 units
Sigma = 71.59 units
Deviation
Squared
independent Demand Ordering Systems
281
11.4. Given the following data, calculate the average demand and the standard deviation.
Period
Actual
Demand
1
1700
2
2100
3
1900
4
2200
5
2000
6
1800
7
2100
8
2300
9
2100
10
1800
Deviation
Deviation
Squared
Total
11.5. If sigma is 130 units, and the demand during the lead time is 250 units, calculate the
safety stock and order point for:
a. A 50% service level.
b. An 85% service level.
Use the table in Figure 11.5 as help to calculate your answer.
a. Safety stock = zero
Order point = 250 units
b. Safety stock = 135 units
Order point = 385 units
11.6. The standard deviation of demand during the lead time is 100 units.
Answer.
a. Calculate the safety stock required for the following service levels: 75%, 80%,
85%, 90%, 95%, and 99.99%.
b. Calculate the change in safety stock required to increase the service levels from 75
to 80%, 80 to 85%, 85 to 90%, 90 to 95%, and 95 to 99.99%. What conclusion do
you reach?
11.7. For an SKU, the standard deviation of demand during the lead time is 150 units, the
annual demand is 10,000 units, and the order quantity is 750 units. Management
says it will tolerate only one stockout per year. What safety stock should be carried?
What is the average inventory? If the lead time is 2 weeks, what is the order point?
Answer.
Safety stock 213 units
Average inventory = 588 units
Order point 598 units
282
Chapter eleven
11.8. A company stocks an SKU with a weekly demand of 225 units and a lead time
of 3 weeks. Management will tolerate one stockout per year. If sigma for the lead
time is 175 and the order quantity is 800 units, what is the safety stock, the average
inventory, and the order point?
11.9. A company stocks an SKU with a weekly demand of 600 units and a lead time of
4 weeks. Management will tolerate one stockout per year. If sigma for the lead time
is 100 and the order quantity is 2500 units, what is the safety stock, the average
inventory, and the order point?
11.10. If the standard deviation is calculated from weekly demand data at 100 units, what
is the equivalent sigma for a 3-week lead time?
Answer. 173 units
11.11. If the safety stock for an item is 200 units and the lead time is 3 weeks, what should
the safety stock become if the lead time is extended to 6 weeks?
Answer. 283 units
11.12. If the weekly standard deviation is 140 units, what is it if the lead time is 3 weeks?
11.13. The safety stock on an SKU is set at 220 units. The supplier says it can reduce the
lead time from 8 to 6 weeks. What should be the new safety stock?
Answer. 191 units
11.14. The safety stock on an SKU is set at 500 units. The supplier says it has to increase
the lead time from 4 to 5 weeks. What should be the new safety stock?
11.15. Management has stated that it will tolerate one stockout per year. The forecast of
annual demand for a particular SKU is 100,000 units, and it is ordered in quantities of 10,000 units. The lead time is 2 weeks. Sales history for the past 10 weeks
follows. Calculate:
a. Sigma for the demand history time interval.
b. Sigma for the lead time interval.
c. The service level.
d. The safety stock required for this service level.
e. The order point.
Week
Actual
Demand
1
2100
2
1700
3
2600
4
1400
5
1800
6
2300
7
2200
8
1600
9
2100
10
2200
Total
Deviation
Deviation
Squared
independent Demand Ordering Systems
283
11.16. If in problem 11.15, management said that it is considering increasing the service
level to one stockout every 2 years, what would the new safety stock be? If the
cost of carrying inventory on this item is $10 per unit per year, what is the cost of
increasing the inventory from one stockout per year to one every 2 years?
11.17. The annual demand for an item is 10,000 units, the order quantity is 250, and the
service level is 90%. Calculate the probable number of stockouts per year.
Answer.
4 stockouts per year
11.18. A company that manufactures stoves has one plant and two distribution centers (DCs).
Given the following information for the two DCs, calculate the gross requirements,
projected available, and planned order releases for the two DCs and the gross requirements, projected available, and planned order releases for the central warehouse.
Distribution Center A
Transit Time: 2 weeks
Order Quantity: 100 units
Week
1
2
3
4
5
Gross Requirements
50
50
85
50
110
In Transit
100
Projected Available
75
Planned Order Release
Distribution Center B
Transit Time:
1 week
Order Quantity: 200 units
Week
1
2
3
4
5
Gross Requirements
120
110
115
100
105
In Transit
200
2
3
4
5
Projected Available
50
Planned Order Release
Central Supply
Lead Time:
2 weeks
Order Quantity: 500 units
Week
1
Gross Requirements
Scheduled Receipts
Projected Available
400
Planned Order Release
Answer.
Planned order release from central supply: 500 in week 2.
284
Chapter eleven
11.19. A company that manufactures snow shovels has one plant and two distribution
centers (DCs). Given the following information for the two DCs, calculate the
gross requirements, projected available, and planned order releases for the two DCs
and the gross requirements, projected available, and planned order releases for the
central warehouse.
Distribution Center A
Transit Time:
2 weeks
Order Quantity: 500 units
Week
1
2
3
4
5
Gross Requirements
300
200
150
275
300
In Transit
500
Projected Available
200
Planned Order Release
Distribution Center B
Transit Time:
2 weeks
Order Quantity: 200 units
Week
1
2
3
4
5
Gross Requirements
50
75
100
125
150
1
2
3
4
5
In Transit
Projected Available
150
Planned Order Release
Central Supply
Lead Time:
1 week
Order Quantity: 600 units
Week
Gross Requirements
Scheduled Receipts
Projected Available
Planned Order Release
400
independent Demand Ordering Systems
285
11.20. A firm orders a number of items from a regional warehouse every 2 weeks. Delivery
takes 1 week. Average demand is 200 units per week, and safety stock is held at
2 weeks’ supply.
a. Calculate the target level.
b. If 600 units are on hand, how many should be ordered?
Answer.
Target level = 1000 units
Order quantity = 400 units
11.21. A regional warehouse orders items once a week from a central warehouse. The
truck arrives 3 days after the order is placed. The warehouse operates 5 days a
week. For a particular brand and size of chicken soup, the demand is fairly steady
at 20 cases per day. Safety stock is set at 2 days’ supply.
a. What is the target level?
b. If the quantity on hand is 90 cases, how many should be ordered?
11.22. A small hardware store uses a periodic review system to control inventory and
calculate order quantities. Inventory is reviewed every 2 weeks and there is a
1-week lead time for delivery. Safety stock is set at 1 week’s supply. The following
table shows the products and the current on-hand inventory. Calculate the target
level and order quantity for each of the items. Use 50 weeks per year for your
calculations.
Item
Annual
Demand
Target
Level
On-Hand
Nut – 6 mm
500
22
Nut – 8 mm
750
54
Bolt – 6 mm
200
0
Bolt – 8 mm
100
6
Screw #8 – 30 mm
250
12
Screw #8 – 40 mm
200
8
Washer – 8 mm
380
20
Washer – 10 mm
100
5
Pin – Split
400
40
Order
Quantity
11.23. Using the data from problem 11.22, calculate the order quantities that would result
if the store switched to a weekly review of inventory. What effect does more
frequent ordering have on order quantities?
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caSe Study 11.1
Carl’s Computers
There was no question about Carl’s genius. Seven years ago he decided to enter the
competitive nightmare that the personal computer business had become. Although on the
surface that appeared to be a rather non-genius-like move, the genius came in the unique
designs and features that he developed for his computer. He also figured a way to promise
delivery in only two days for the local and regional market. Other computer makers also
had rapid production and delivery, but they were national competitors, and the delivery
time from distant locations generally made Carl able to outcompete them on delivery.
Carl soon had a loyal following, especially among the many small businesses in
the area. Not only could Carl deliver quickly, but he also had very rapid service to deal
with any technical problems. That service feature became critical for the local businesses
whose very livelihood depended on the computers, and soon that rapid service capability
became more important than the initial product delivery. Since most of these businesses
were fairly small, they could not afford to have their own in-house computer experts, so
they depended heavily on Carl.
the Current Situation
All was not totally rosy at Carl’s Computers, however. Recently they had hired Rosa
Chang for the newly developed position of inventory manager for aftermarket service.
In the first week Rosa got a good idea of the challenges facing her after she interviewed
several of the people at Carl’s.
randy smith, customer service manager: “I’m not sure what you need to do, but
whatever it is needs to be done fast! At this point our main competitive edge
other than product delivery is service response, and I’m always hearing that we
can’t get a unit in the field serviced because some critical part is missing. Both
the customers and the field service people are complaining about it. They make
a service call, find out they need a certain part, but in many cases we’re out of
the part. The customers tend to be fairly loyal, but their patience is wearing
thin—our policy is to provide at least a 98% customer service level, and we’re
not even close. That’s not the only problem, though. Since our service is declining, the customers are looking more closely at our prices. I’d like to cut them a
break, but our financial people tell us our margins are already too thin, and get
this—one major reason is that our inventory and associated inventory costs are
too high! It looks to me as if we have a very large amount of the wrong stuff
here. I don’t know that for sure, but I sure hope you can find a solution, and
fast!”
ellen bedrosian, chief engineer: “Boy, am I glad you’re here! The inventory
problems are killing us in engineering. Carl’s has always been known for unique
designs, and we’ve been trying hard to keep ahead of the competitive curve on
that issue. The problem is that most of the time when we push hard to get a new
design out, the inventory and financial people tell us we have to wait. It seems
like they always have too much of the old design inventory around, and the
financial ‘hit’ to make it immediately obsolete would be too severe. We’re told
that as soon as we announce a new design, many of our customers would want
it, so that tends to make most existing old design material—even for service—
obsolete. We try to tell the service inventory people when we have a new design
coming so they can use up the old material, but somehow it never seems to
work out.”
jim hughes, purchasing manager: “Well, Rosa, I wish you luck—you’ll need it.
I’m getting pressure from so many directions, sometimes I don’t know how to
respond. First, the financial people are always telling me to cut or control costs.
The engineers then are always coming out with new designs, most of which
represent purchased parts. A lot of our time is spent working with suppliers on
independent Demand Ordering Systems
287
the new designs, while trying to get them to have very rapid delivery with low
prices. Although most can live with that, where we really jerk them around is
with the changes in orders. One minute our field service people tell us they’ve
run out of something and they need delivery immediately. In many cases they
don’t even have an order for that part on the books. The next thing you know
they want us to cancel an order for something that only a day before they said
was critical. Our buyers and suppliers are good, but they’re not miracle workers
and they can’t do everything at once. Some of our suppliers are even threatening to refuse our business if we don’t get our act together. We’ve tried to offer
solutions for the field service people, but nothing seems to work. Maybe they
just don’t care.”
mary shoulton, chief financial officer: “If you can help us with this inventory problem, you’ll be well worth your salary, and then some! Here we are
being competitively crunched for price, delivery, and efficient service, and our
service inventory costs seem to have gone completely out of control. The total
inventory has climbed more than 200% in the last two years, while our service
revenues have only grown 15%. On top of that, we have had an increase in
obsolete material write-off of 80% in that same two-year period. In addition,
significant inventory-related costs have come from expediting. Premium freight
shipments, such as flying in parts, caused by critical part shortages cost us over
$67,000 last year alone. Do you realize that represents almost 20% of our gross
profit margin from the service business? With our interest rates, warehousing,
and obsolete inventory costs, we recognize a 23% inventory holding cost. Given
our huge inventory level, that takes another big bite out of profits. All this suggests to me we need to get control of the situation or we may find ourselves out
of business!”
franklin knowles, field service supervisor: “Until they hired you, the other
production supervisor and I had been in charge of inventory. I hate to discourage you, but it looks like an impossible job. The purchasing people bought a
bunch of standard-size bins, and they told us that as soon as we had a week’s average part usage for each part, we should order more—specifically, ‘enough to
fill up the bin.’ Since most of their lead times were a week or less, it sure made
sense. All the records were kept on computer, therefore the computer could be
programmed to tell us when we had only the week’s supply. It made great sense
to me, but something kept going wrong. First, field service technicians seemed
to frequently grab parts without filling out a transaction. That made our records
go to pot. As a matter of fact, we had a complete physical inventory a couple of
months ago, and it showed our records to be less than 30% accurate! I suspect
our records are almost that bad again, and we don’t have another physical inventory scheduled for another nine months.
“Second, with our records so bad, the field service technicians can never
tell if we really have the parts or not. Several of them have started to take large
quantities of critical parts and are keeping their own inventory. When it comes
time to replace their own ‘private stock,’ they take a bunch more. That has
made the demand on the central inventory appear very erratic. One day we have
plenty, and the next day we’re out! You can imagine how happy purchasing is
when the first time they see a purchase order that is requesting an immediate
urgent shipment. We’ve made a policy that the technicians are only supposed
to have a few specifically authorized parts with them, but I’m sure many of the
technicians are violating that policy big time.”
quentin bates, field service technician: “Something is drastically wrong with
our inventory, and it’s driving me and the other techs crazy. We’re not supposed
to keep much inventory with us, only a few commonly used parts. If we have a
field problem requiring a part, we’re supposed to be getting it from the central
inventory. Problem is, much of the time it’s not there. We have to take time to
pressure purchasing for it, and then have to try to calm our customers while we
wait for delivery. In the meantime, the customers’ systems are often unusable,
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and they’re losing business. It doesn’t take too long before they’re really mad
at us. I guess the people at purchasing don’t care, since we have to take all the
heat. Lately, I’ve been taking and keeping a bunch of parts I’m not really supposed to have in my inventory, and I know the other field technicians do as well.
That’s saved us a few times, but the situation seems to be getting worse.”
Now that Rosa had some real information as to the nature of the problems, she needed
to start developing solutions—and it appeared that it was important to come up with good
solutions fast! The first thing she tried to do was take a couple of part numbers at random
and see if she could improve on the ordering approach.
The first number she selected was the A233 circuit board. The average weekly usage
was 32. The lead time was given as one week. The board cost $18, and the cost to place
an order was given as $16. The quantity ordered to fill the bin was usually 64. The second
number was the P656 power supply. It cost $35, but since the supplier only required a fax
to order the cost was only $2 per order. Even with the fax, the delivery lead time was two
weeks. The average weekly demand for the power supply was 120. The company typically
ordered 350 units at a time. Recently, the supplier for the circuit board hinted that it might be
able to give Carl’s a price break of $2 per board if Carl’s would order 200 or more at a time.
assignment
1. Using the data on the two part numbers given, provide a comprehensive evaluation of
the ordering policies. Compare the present annual average cost with the cost of using
a system such as EOQ, and discuss any other order policies as appropriate.
2. Should Carl’s pursue the price break? Why or why not?
3. What do you think the sources of the other problems are? Be specific and analyze as
completely as possible.
4. Develop a comprehensive plan to help Rosa get the inventory back under control.
Chapter
twelve
Physical inventory and
warehouse ManageMent
introduction
Because inventory is stored in warehouses, the physical management of inventory and
warehousing are intimately connected. In some cases, inventory may be stored for an
extended time. In other situations, inventory is turned over rapidly, and the warehouse
functions as a distribution center.
This chapter will deal with the physical management of inventory in a warehouse,
including basic approaches to warehouse layout, the activities involved in handling goods,
and the controls necessary to work efficiently while maintaining a desired level of customer service. Inventory accuracy is the responsibility of warehousing, and methods of
determining the accuracy of stocks will be covered, along with a description of how to
conduct an annual physical audit. The cycle counting method of auditing inventory will
show the advantages of timely correction of errors, along with improved error prevention.
Bar coding and radio frequency identification (RFID) will be introduced as a means to
improve the speed and accuracy of gathering information.
In a factory, stores, or stockrooms, perform the same functions as warehouses and
contain raw materials, work-in-process inventory, finished goods, supplies, and possibly
repair parts. Since they perform the same functions, stores and warehouses are treated
alike in this chapter.
warehousing ManageMent
As with other elements in a distribution system, the objective of a warehouse is to minimize cost and maximize customer service. To do this, efficient warehouse operations
perform the following:
Provide timely customer service.
■■ Keep track of items so they can be found readily and correctly.
■■ Minimize the total physical effort and thus the cost of moving goods into and out of
storage.
■■ Provide communication links with customers.
■■
The costs of operating a warehouse can be broken down into capital and operating
costs. Capital costs are dependent on the type of material to be stored and include those of
building space, racking or shelving, tanks or containers, and material handling equipment.
The space needed depends on the peak quantities that must be stored, the methods of storage, and the need for ancillary space for aisles, docks, offices, and so on.
The major operating cost is labor, and the measure of labor productivity is the number
of units, e.g., pallets, that an operator can move in a day. The labor cost depends on the
type of material handling equipment used, the location and accessibility of stock, warehouse layout, stock location system, and the order picking system used.
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Warehouse Activities
Operating a warehouse involves several processing activities, and the efficient operation of
the warehouse depends upon how well these are performed. These activities are as follows:
1. Receive goods. The warehouse accepts goods from outside transportation or an
attached factory and accepts responsibility for them. This means the warehouse must:
a. Check the goods against an order and the bill of lading.
b. Check the quantities.
c. Check for damage and fill out damage reports if necessary.
d. Inspect goods if required.
2. Identify the goods. Items are identified with the appropriate stockkeeping unit
(SKU) number (part number) and the quantity received is recorded.
3. Dispatch goods to storage. Goods are sorted and put away.
4. Hold goods. Goods are kept in storage and under proper protection until needed.
5. Pick goods. Items required from stock must be selected from storage and brought to
a consolidating area.
6. Consolidate the shipment. Goods making up a single order are brought together and
checked for omissions or errors. Order records are updated.
7. Dispatch the shipment. Orders are packaged, shipping documents prepared, and
goods loaded on the right vehicle.
8. Record the information. A record must be maintained for each item in stock showing the quantity on hand, quantity received, quantity issued, and location in the warehouse. The system can be very simple, depending on a minimum of written information and human memory, or it may be a sophisticated computer-based system.
In various ways, all these activities take place in any warehouse. The complexity
depends on the number of SKUs handled, the quantities of each SKU, and the number of
orders received and filled. To maximize productivity and minimize cost, warehouse management must work with the following:
1. Maximum use of space. Usually the largest capital cost is for space. This means not
only floor space but cubic space as well since goods are stored in the space above the
floor as well as on it.
2. Effective use of labor and equipment. Material handling equipment represents the
second-largest capital cost and labor the largest operating cost. There is a trade-off
between the two in that labor costs can be reduced by using more material handling
equipment. Warehouse management will need to:
■■ Select the best mix of labor and equipment to maximize the overall productivity of
the operation.
■■ Provide ready access to all SKUs. The SKUs should be easy to identify and find.
This requires a good stock identification system, stock location system, and
layout.
■■ Move goods efficiently. Most of the activity that goes on in a warehouse is material handling: the movement of goods into and out of stock locations.
Several factors influence effective use of warehouses. Some are:
Cube utilization and accessibility.
■■ Stock location.
■■ Order picking and assembly.
■■ Packaging.
■■
With the exception of packaging, these are discussed in the following sections.
Physical Inventory and Warehouse Management
291
Cube Utilization and Accessibility
Goods are stored not just on the floor, but also in the cubic space of the warehouse.
Although the size of a warehouse can be described as so many square feet, warehouse
capacity depends on how high goods can be stored and the density of the storage.
Space is also required for aisles, receiving and shipping docks, offices, and order
picking and assembly. In calculating the space needed for storage, some design figure for
maximum inventory is needed. Suppose that a maximum of 90,000 cartons is to be inventoried and 30 cartons fit on a pallet. Space is needed for 3000 pallets. If pallets are stacked
three high, 1000 pallet positions are required. A pallet is a platform usually measuring
48″ * 40″ * 4″.
Pallet positions Suppose a section of a warehouse is as shown in Figure 12.1. Since
the storage area is 48″ deep, the 40″ side is placed along the wall. The pallets cannot be
placed tight against one another; a 2″ clearance must be allowed between them so they can
be moved. This then leaves room for 1120′ * 12″2 , 42″ = 34.3, or 34, pallet positions along each side of the aisle. Since the pallets are stacked three high, there is room for
204 (34 * 3 * 2) pallets.
Example Problem
A company wants to store an SKU consisting of 13,000 cartons on pallets, each
containing 30 cartons. How many pallet positions are needed if the pallets are stored
three high?
Answer
Number of pallets required = 13,000 , 30 = 433.33 S 434 pallets
Number of pallet positions = 434 , 3 = 144.67 S 145 pallet positions
Notice one pallet position will contain only two pallets.
Accessibility Accessibility means being able to get at the goods wanted with a minimum amount of work. For example, if no other goods had to be moved to reach an SKU,
the SKU would be 100% accessible. As long as all pallets contain the same SKU, there is
no problem with accessibility. The SKU can be reached without moving any other product. When several SKUs are stored in the area, each product should be accessible with a
minimum of difficulty.
Cube utilization Suppose items are stacked along a wall, as shown in Figure 12.2.
There will be excellent accessibility for all items except item 9, but cube utilization
is not maximized. Cube utilization is the use of space horizontally and vertically.
There is room for 30 pallets, but only 21 spaces are being used for a cube utilization of
70%: 121 , 302 * 100%. Some method must be devised to increase cube utilization
and maintain accessibility. One way is to install tiers of racks so that lower pallets can
be removed without disturbing the upper ones. This represents a trade-off between
the capital cost of the racking and the savings in the operating cost of extra handling.
Whether the additional cost is worthwhile will depend on the amount of handling and
the savings involved.
120'
4'
Aisle
4'
FIgurE 12.1 Cube utilization.
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Chapter twelve
1
1
1
1
1
1
2
2
2
3
3
3
4
4
4
5
6
7
8
10
9
FIgurE 12.2 Cube utilization versus accessibility.
Example Problem
A small warehouse stores five different SKUs in pallet loads. If pallets are stacked three
high and there is to be 100% accessibility, how many pallet positions are needed? What
is the cube utilization?
SKU A
4 pallets
SKU B
6 pallets
SKU C
14 pallets
SKU D
8 pallets
SKU E
5 pallets
Total
37 pallets
Answer
SKU
Pallet positions
A: 4 pallets
2
B: 6 pallets
2
C: 14 pallets
5
D: 8 pallets
3
E: 5 pallets
2
Total
14
In 14 pallet positions, there is room to store 14 * 3 = 42 pallets.
Number of pallets actually stored = 37
Cube utilization = 137 , 422 * 100% = 88%
Stock Location
Stock location, or warehouse layout, is concerned with the location of individual items
in the warehouse. There is no single universal stock location system suitable for all
occasions, but there are a number of basic systems that can be used. Which system, or
mix of systems, is used depends on the type of goods stored, the type of storage facilities needed, the throughput, and the size of orders. Whatever the system, management
must maintain enough inventory of safety and working stock to provide the required
level of customer service, keep track of items so they can be found easily, and reduce
the total effort required to receive goods, store them, and retrieve them for shipment.
The following are some basic systems of locating stock:
Group functionally related items together. Group together items similar in their
use (functionally related). For example, put all hardware items in the same area of the
warehouse. If functionally related items are ordered together, order picking is easier.
Warehouse personnel become familiar with the locations of items.
■■ Group fast-moving items together. If fast-moving items are placed close to the
receiving and shipping area, the work of moving them in and out of storage is reduced.
Slower-moving items can be placed in more remote areas of the warehouse.
■■ Group physically similar items together. Physically similar items often require their
own particular storage facilities and handling equipment. Small packaged items may
require shelving, whereas heavy items, such as tires or drums, require different facilities and handling equipment. Frozen foods need freezer storage space.
■■ Locate working stock and reserve stock separately. Relatively small quantities of
working stock, stock from which withdrawals are made, can be located close to the
■■
Physical Inventory and Warehouse Management
293
consolidating and shipping area, whereas reserve stock used to replenish the working stock can be located more remotely. This allows order picking to occur in a compact area and replenishment of the working stock in bulk by pallet or container load.
Warehouses with racking often use the floor level for order picking with reserve stock
stored in the upper levels.
There are two basic systems for assigning specific locations to individual stock items:
fixed location and random location. Either system may be used with any of the location
systems cited in the preceding paragraphs.
Fixed location In a fixed-location system, an SKU is assigned a permanent location
or locations, and no other items are stored there. This system makes it possible to store
and retrieve items with a minimum of recordkeeping. In some small, manual systems, no
records are kept at all. It is like always keeping cornflakes on the same shelf in the kitchen
cupboard at home. Everything is nice and simple so things are readily found. However,
fixed-location systems usually have poor cube utilization. If demand is uniform, presumably the average inventory is half the order quantity, and enough space has to be allocated
for a full-order quantity. On the average, only 50% of the cube space is utilized. Fixedlocation systems are often used in small warehouses where space is not at a premium,
where throughput is small, and where there are few SKUs. Fixed location systems have
particularly poor space utilization when product design changes are frequent and yet stock
of the older design must be maintained for a period of time.
Random location (sometimes called floating location) In a random-location
system, goods are stored wherever there is appropriate space for them. The same SKU
may be stored in several locations at the same time and different locations at different
times. The advantage to this system is improved cube utilization. However, it requires
accurate and up-to-date information on item location and the availability of empty storage
space so items can be put away and retrieved efficiently. If either part number or location
information is incorrect, then someone looking for the material will have a major problem
trying to locate the material, as it could be anywhere in the warehouse. Modern warehouses using random location systems are usually computer-based. The computer assigns
available locations to incoming items, records what items are on hand and where they are
located, and directs the order picker to the right location to find the item. Thus, cube utilization and warehouse efficiency are greatly improved.
Over time, other storage systems have evolved to address special cases or problems
with the two basic location systems.
Zone random storage This is really a hybrid system of the two basic systems, combining advantages and some disadvantages of both. First, zones are established in the
warehouse where closely related products are stored. Then within each zone, the material
can be randomly located as in a floating location system. For example, if a zone is established for locating all fasteners and if a location or part number for one fastener is incorrectly placed in the record, then the entire warehouse does not need to be searched, only
the zone for fasteners. Because of the floating storage within the zone, the system is fairly
efficient with good cube utilization.
Point-of-use storage Sometimes, particularly in repetitive manufacturing and in a
lean production environment, inventory is stored close to where it will be used. There are
several advantages to point-of-use storage.
Materials are readily accessible to users.
Material handling is reduced or eliminated.
■■ Central storage costs are reduced.
■■ Material is accessible at all times.
■■
■■
This method is excellent as long as inventory is kept low and operating personnel can
keep control of inventory records. Sometimes C items are issued as floor stock, where
manufacturing is issued a large quantity, which is used as needed. Inventory records are
adjusted when the stock is issued, not when it is used.
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Central storage As opposed to point-of-use storage, central storage contains all
inventory in one central location. There are several advantages:
Ease of control.
■■ Inventory record accuracy is easier to maintain.
■■ Specialized storage can be used.
■■ Reduced safety stock, since users do not need to carry their own safety stock.
■■
Order Picking and Assembly
Once an order is received, the items on the order must be retrieved from the warehouse,
assembled, and prepared for shipment. All these activities involve labor and the movement of goods. The work should be organized to provide the level of customer service
required and at least cost. There are several systems that can be used to organize the work,
among which are the following:
1. Area system. The order picker circulates throughout the warehouse selecting the items
on the order, much as a shopper would in a supermarket. The items are then taken to the
shipping area for shipment. The order is self-consolidating in that when the order picker
is finished, the order is complete. This system is generally used in small warehouses
where goods are stored in fixed locations.
2. Zone system. The warehouse is divided into zones, and order pickers work only
in their own area. An order is divided up by zone, and each order picker selects
those items in their zone and sends them to the consolidating area, where the
order is assembled for shipment. Each order is handled separately and leaves the
zone before another is handled. If applied to the supermarket example above, one
individual would go to say the fresh fruit department, another to the freezer section, and so on. They would then consolidate the items from each department at
the checkout.
Zones are usually established by grouping together related parts. Parts may be related because of the type of storage needed for them, e.g., freezer storage, or because
they are often ordered together.
A variation of the zone system is to have the order move to the next zone rather than
to the consolidating area. By the time it exits the last zone, it is assembled for shipment.
3. Multi-order system. This system is the same as the zone system except that rather
than handling individual orders, a number of orders are gathered together and all the
items divided by zone. The pickers then circulate through their area, collecting all the
items required for that group of orders. The items are then sent to the consolidating
area where they are sorted to individual orders for shipment.
Kitting saves tiMe when it is needed the Most
In 2014, Supply Logistics, a division of New York City
Transit, which is the operator of the city’s bus and subway
systems, issued nearly 2.5 million line items from its
network of 65 storerooms. This material is used in the
maintenance and repair of over 5600 buses and 6500
subway cars. With the average bus or subway storeroom
less than 5000 sq. ft., space is always at a premium. While
Supply Logistics storerooms utilize a variety of space-saving strategies (mezzanines, vertical carousels, specialized
racks for glass, pipe, hoses, etc.), labor is another limiting
constraint. Too many people in a small space receiving and
issuing material simultaneously can become counterproductive and could pose a serious safety hazard.
One way that Supply Logistics has maximized space
in the storerooms is by repackaging material into kits. The
kitting operation began in 1989. In 2001 it reached the
milestone of 1 million kits assembled and by 2014 it had
assembled 3 million. In 2014 alone Supply Logistics assembled 216,000 kits.
What is the impact of kitting to New York City Transit?
The average kit contains 12 line items. If these kits had
not been produced that would mean that Supply Logistics
would have had to issue an additional 2.4 million lines from
its storerooms, nearly doubling the current annual issues.
This would have resulted in a much larger investment in
storeroom space and equipment, as well as hiring additional
staff. Kitting has saved time and money for Supply Logistics,
New York City Transit, and the citizens of New York City.
Submitted by:
Gary A. Smith, CFPIM, CSCP
Vice President, Division of Supply Logistics
New York City Transit
Physical Inventory and Warehouse Management
295
The area system is simple to manage and control, but as the warehouse throughput
and size increases, it becomes unwieldy. The zone systems break down the order-filling
process into a series of smaller areas that can be better managed individually. The multiorder system is probably most suited to the situation in which there are many items or
many small orders with few items.
Working stock and reserve stock In addition to the systems mentioned, reserve
stock and working stock may be separated. This is appropriate when the pick unit for a
customer’s order may be a box or a case that is stored on pallets. A pallet can be moved
into the working area by a lift truck and cartons or boxes picked from it. The working
stock is located close to the shipping area so the work in picking is reduced. A separate
workforce is used to replenish the working stock from the reserve stock.
Physical control and security
Because inventory is tangible, items have a nasty habit of becoming lost, stray, or stolen,
or of disappearing in the night. It is typically not that people are dishonest, rather that they
are forgetful. What is needed is a system that makes it difficult for people to make mistakes or be dishonest. There are several elements that help:
A good-part numbering system. Part numbering was discussed in Chapter 4. It must
be clear and easy to use for the order pickers and material handlers.
■■ A simple, well-documented transaction system. When goods are received, issued, or
moved in any way, a transaction occurs. There are four steps in any transaction: identify
the item, verify the quantity, record the transaction, and physically execute the transaction.
■■
1. Identify the item. Many errors occur because of incorrect identification. When
receiving an item, the purchase order, part number, and quantity must be properly
identified. When goods are stored, the location must be accurately specified. When
issued, the quantity, location, and part number must be recorded.
2. Verify quantity. Quantity is verified by a physical count of the item by weighing
or by measuring. Sometimes standard-sized containers are useful in counting.
3. Record the transaction. Before any transaction is physically carried out, all information about the transaction must be recorded.
4. Physically execute the transaction. Move the goods in, about, or out of the storage area.
Limited access. Inventory must be kept in a safe, secure place with limited general access.
It should be locked except during normal working hours. This is less to prevent theft than
to ensure people do not take things without completing the transaction steps. If people can
wander into the stores area at any time and take something, the transaction system fails.
■■ A well-trained workforce. Not only should the stores staff be well trained in handling and storing material and in recording transactions, but other personnel who interact with stores must be trained to ensure transactions are recorded properly.
■■
inventory record accuracy
The usefulness of inventory is directly related to its accuracy. Based on the inventory
record, a company determines net requirements for an item, releases orders based on
material availability, and performs inventory analysis. If the records are not accurate, there
will be shortages of material, disrupted schedules, late deliveries, lost sales, low productivity, and excess inventory of the wrong items.
These three pieces of information must be accurate: part description (part number),
quantity, and location. Accurate inventory records enable firms to:
Operate an effective materials management system. If inventory records are inaccurate, gross-to-net calculations will be in error.
■■ Maintain satisfactory customer service. If records show an item is in inventory
when it is not, any order promising of that item will be in error.
■■
296
Chapter twelve
Operate effectively and efficiently. Planners can plan, confident that the parts will
be available.
■■ Analyze inventory. Any analysis of inventory is only as good as the data it is based on.
■■
Inaccurate inventory records will result in the following, with resulting costs to the
company.
Lost sales.
Shortages and disrupted schedules.
■■ Excess inventory (of the wrong things).
■■ Low productivity.
■■ Poor delivery performance.
■■ Excessive expediting, since people will always be reacting to a bad situation rather
than planning for the future.
■■
■■
Causes of Inventory Record Errors
Poor inventory record accuracy can be caused by many things, but they all result from
poor recordkeeping systems and poorly trained personnel. Some examples of causes of
inventory record error follow:
Unauthorized withdrawal of material. Employees will often take items from inventory due to problems or shortages in their department. This usually solves a problem
for the department but the transactions are often not recorded.
■■ Unsecured stockroom. Secure stockrooms prevent unauthorized withdrawal, which
may or may not be legitimate.
■■ Poorly trained personnel. Most employees do not realize the consequences of not
properly recording transactions or picking the wrong items.
■■ Inaccurate transaction recording. Errors can occur because of inaccurate piece
counts, unrecorded transactions, delay in recording transactions, inaccurate material
location, and incorrectly identified parts.
■■ Poor transaction recording systems. Most systems today are computer-based and can
provide the means to record transactions properly. Errors, when they occur, are usually
the fault of human input to the system. The documentation reporting system should be
designed to reduce the likelihood of human error.
■■ Lack of audit capability. Some program of verifying the inventory counts and locations is necessary. The most popular one today is cycle counting, discussed in the next
section.
■■
Measuring Inventory Record Accuracy
Inventory accuracy ideally should be 100%. Banks and other financial institutions reach
this level, but other companies can move toward this potential.
Figure 12.3 shows 10 inventory items, their physical count, and the quantity shown
on their record. What is the inventory accuracy? The total of all items is the same, but only
2 of the 10 items are correct. Is the accuracy 100% or 20% or something else?
Tolerance To judge inventory accuracy, a tolerance level for each part must be specified.
For some items, this may mean no variance; for others, it may be very difficult or costly to
measure and control to 100% accuracy. An example of the latter might be nuts or bolts ordered
and used in the thousands. For these reasons, tolerances are set for each item. Tolerance is the
amount of permissible variation between an inventory record and a physical count.
Tolerances are set on individual items based on value, critical nature of the item,
availability, lead time, ability to stop production, safety problems, or the difficulty of getting precise measurement.
Figure 12.4 shows the same data as the previous figure but includes tolerances. This
information tells us exactly what inventory accuracy is.
Physical Inventory and Warehouse Management
Part Number
Inventory
Record
Shelf Count
1
100
105
2
100
100
3
100
98
4
100
97
5
100
102
6
100
103
7
100
99
8
100
100
9
100
97
10
100
99
Total
1000
1000
297
FIgurE 12.3 Inventory record accuracy.
Part
Number
Inventory
Record
Shelf
Count
Tolerance
Within
Tolerance
1
100
105
5%
X
2
100
100
+
– 0%
X
3
100
98
+
– 3%
X
4
100
97
+
– 2%
5
100
102
+
– 2%
6
100
103
+
– 2%
7
100
99
+
– 3%
X
8
100
100
+
– 0%
X
9
100
97
+
– 5%
X
10
100
99
+
– 5%
X
Total
1000
1000
FIgurE 12.4 Inventory accuracy with tolerance.
Outside
Tolerance
X
X
X
298
Chapter twelve
Example Problem
Determine which of the following items are within tolerance. Item A has a tolerance of
;5%; item B, ;2%; item C, ;3%; and item D, ;0%.
Part
Number
Shelf
Count
Inventory
Record
Tolerance
A
1500
1550
+
– 5%
B
120
125
+
– 2%
C
225
230
+
– 3%
D
155
155
+
– 0%
Answer
Item A.
Item B.
Item C.
Item D.
With a tolerance of ;5%, variance can be up to ;75 units.
Item A is within tolerance.
With a tolerance of ;2%, variance can be up to ;2 units.
Item B is outside tolerance.
With a tolerance of ;3%, variance can be up to ;7 units.
Item C is within tolerance.
With a tolerance of ;0%, variance can be up to ;0 units.
Item D is within tolerance.
Auditing Inventory Records
Errors occur, and they must be detected so inventory accuracy is maintained. There
are two basic methods of checking the accuracy of inventory records: periodic (usually
annual) counts of all items and cyclic (usually daily or weekly) counts of specified items.
It is important to audit record accuracy, but it is more important to audit the record system
to find the causes of inaccuracies and eliminate them. Cycle counting does this; periodic
audits tend not to.
Periodic (annual) inventory The primary purpose of a periodic (annual) inventory
is to satisfy the financial auditors that the inventory records represent the value of the
inventory. To planners, the physical inventory represents an opportunity to correct any
inaccuracies in the records. Whereas financial auditors are concerned with the total value of
the inventory, planners are concerned with item detail.
The responsibility for taking the physical inventory usually rests with materials management and finance, who should ensure that a good plan exists and it is followed. George
Plossl once said that taking a physical inventory was like painting; the results depend on
good preparation.1 There are three factors in good preparation: housekeeping, identification, and training.
Housekeeping Inventory must be sorted and the same parts collected together so they
can easily be counted. Sometimes items can be pre-counted and put into sealed cartons.
Identification Parts must be clearly identified and tagged with part numbers. This can,
and should, be done before the inventory is taken. Personnel who are familiar with parts
identification should be involved and all questions resolved before the physical inventory
starts.
1
George W. Plossl, Production and Inventory Control, Principles and Techniques, 2nd ed., Appendix VI:
Physical Inventory Techniques. Englewood Cliffs, NJ: Prentice Hall, 1985.
Physical Inventory and Warehouse Management
299
Training Those who are going to do the inventory must be properly instructed and
trained in taking inventory. Physical inventories are usually taken once a year, and the
procedure is not always remembered from year to year.
Process Taking a physical inventory consists of four steps:
1. Count items and record the count on a ticket left on the item.
2. Verify this count by recounting or by sampling.
3. When the verification is finished, collect the tickets and list the items in each
department.
4. Reconcile the inventory records for differences between the physical count and inventory dollars. Financially, this step is the job of accountants, but materials personnel
are involved in adjusting item records to reflect what is actually on hand. If major
discrepancies exist, they should be checked immediately.
Taking a physical inventory is a time-honored practice in many companies mainly
because it has been required for an accurate appraisal of inventory value for the annual
financial statements. However, taking an annual physical inventory presents several
problems. Usually, the factory has to be shut down, thus losing production time; labor
and paperwork are expensive; the job is often done hurriedly and poorly since there is
much pressure to get it done and the factory running again. In addition, the people doing
the inventory are not used to the job or the inventory and are prone to making errors. As a
result, more errors often are introduced into the records than are eliminated.
Because of these problems, the idea of cycle counting has developed.
Cycle counting Cycle counting is a system of counting inventory continually
throughout the year. Physical inventory counts are scheduled so that each item is counted
on a predetermined schedule. Depending on their importance, some items are counted
frequently throughout the year whereas others are not. The idea is to count selected items
each day.
The advantages to cycle counting are as follows:
Timely detection and correction of problems. The purpose of the count is first to find
the cause of error and then to correct the cause so the error is less likely to happen again.
■■ Complete or partial reduction of lost production. Cycle counting can usually be
done without stopping production.
■■ Use of personnel trained and dedicated to cycle counting. This provides experienced
inventory takers who will not make the errors personnel involved in the physical inventory do. Cycle counters are often also trained to identify problems and to correct them.
Sometimes the people within production, who use the parts, are also the cycle counters.
■■
It should be noted that while cycle counting is an important method to keep records
accurate with minimal disruption of production, it is even more important to use the
results of the cycle counts to discover the causes of the record inaccuracy. As causes of
inaccuracy are discovered and process improvement in the transaction system address the
causes, not only are the records more accurate but over time inaccuracies become very
rare. In some organizations, cycle counters who also are responsible for finding causes of
inaccuracy become highly valued personnel in the company, since the activity of evaluating processes to discover the cause of a problem makes them highly knowledgeable
in both the company and in the approaches to process improvement. This activity is an
important part of data governance, which is the set of policies, procedures, and standards
for maintaining effective data and information systems.
In some organizations with highly effective cycle count programs, the financial auditors are encouraged to audit the records as kept through cycle counting rather than the
annual physical inventory. If a high enough confidence level is reached by the financial
auditors, they may decide to forgo the annual physical inventory, thereby saving the company substantial money and time.
300
Chapter twelve
Classification
Number
of Items
Count
Frequency
per Year
Number
of Counts
per Year
% of Total
Counts
Counts
per Day
A
B
C
1000
1500
2500
12
4
1
12,000
6000
2500
58.5
29.3
12.2
48
24
10
Total counts per year
Workdays per year
Counts per day
20,500
250
82
FIgurE 12.5 Scheduling cycle counts.
Count frequency The basic idea is to count some items each day so all items are
counted a predetermined number of times each year. The number of times an item is
counted in a year is called its count frequency. For an item, the count frequency should
increase as the value of the item and number of transactions (chance of error) increase.
Several methods can be used to determine the frequency. Three common ones are the
ABC method, zone method, and location audit system.
■■
ABC method. This is a popular method. Inventories are classified according to the
ABC system (refer to Chapter 9). Some rule is established for count frequency. For
example, A items might be counted weekly or monthly; B items, bimonthly or quarterly; and C items, biannually or once a year. On this basis, a count schedule can be
established. Figure 12.5 shows an example of a cycle count scheduled using the ABC
system. Note that there should be 82 items counted each day, of which 58.5% should
be A items or 48 counts per day. This will result in 250 days * 48 A items per day or
12,000 counts per year.
Example Problem
A company has classified its inventory into ABC items. It has decided that A items are
to be counted once a month; B items, four times a year; and C items, twice a year.
There are 2000 A items, 3000 B items, and 5000 C items in inventory. Develop a
schedule of the counts for each class of item.
Answer
Classification
Number
of Items
Count
Frequency
per Year
Number
of Counts
per Year
% of Total
Counts
Counts
per Day
A
B
C
2000
3000
5000
12
4
2
24,000
12,000
10,000
52.2
26.1
21.7
96
48
40
Total counts per year
Workdays per year
Counts per day
46,000
250
184
Zone method. Items are grouped by zones to make counting more efficient. The system is used when a fixed-location system is used, or when work-in-process or transit
inventory is being counted.
■■ Location audit system. In a floating-location system, goods can be stored anywhere,
and the system records where they are. Because of human error, these locations may
not be 100% correct. If material is mislocated, normal cycle counting may not find it.
In using location audits, a predetermined number of stock locations are checked each
■■
Physical Inventory and Warehouse Management
301
period. The item numbers of the material in each bin are checked against inventory
records to verify stock point locations.
A cycle counting program may include all these methods. The zone method is ideal
for fast-moving items. If a floating-location system is used, a combination of ABC and
location audit is appropriate.
When to count Cycle counts can be scheduled at regular intervals or on special occasions. Some selection criteria are as follows:
When an order is placed. Items are counted just before an order is placed. This has the
advantage of detecting errors before the order is placed and reducing the amount of work
by counting at a time when stock is low. Items with high order frequency have more
opportunities for transaction errors. This method counts fast moving items more often.
■■ When an order is received. Inventory is at its lowest level and easiest to count. Also,
the effort of going to the stock location is already taking place.
■■ When the inventory record reaches zero. Again, this method has the advantage of
reducing work.
■■ When a specified number of transactions have occurred. Errors occur when transactions occur. Fast-moving items have more transactions and are more prone to error.
■■ When an error occurs. A special count is appropriate when an obvious error is
detected. This may be a negative balance on the stock record or when no items can be
found although the record shows some in stock.
■■
consignMent inventory and vendor-Managed
inventory (vMi)
A recent trend in many companies is to essentially outsource some of the inventory control
and holding costs to suppliers. This type of inventory is called consignment inventory.
The supplier typically places inventory in a specified customer location for the customer
to use as needed. Under nonconsignment conditions, when a customer orders material
from a supplier, the customer is obligated to pay the supplier under predefined terms, for
example, within 30 days. With consignment inventory, the customer is not obligated to
pay for the material until after it is used or sold. This clearly has a possible cost reduction
advantage for the customer, since they have no money invested in the inventory, but can
inventory accuracy: “a way of life”
Inventory accuracy has become a way of life for Supply
Logistics at New York City Transit. Each of the 65 storerooms
is cycle counted quarterly while the main warehouses are
cycle counted 6 times per year. Each cycle count consists of
three activities: Stockkeeping Performance (SKP), Bin Stockkeeping Performance (Bin SKP), and Precision, Accuracy &
Control Test (PACT). Cycle counts are done by a third party
contractor to avoid any conflicts.
Stockkeeping Performance is measured by choosing
a number of items to test (fast, medium, and slow movers)
and counting the selected inventory. For example, if 150
items are chosen, then all of the items are counted and the
physical inventory is reconciled with the quantity on hand
in the inventory management system. If two items fail, the
SKP percentage is 98.7% (148/150).
Bin SKP measures the accuracy at the location level.
In the example above, if the 150 items were spread among
160 locations (bins), then the inventory in each is checked
for accuracy. In the current example, if 7 locations failed
to have the correct inventory, the Bin SKP percentage
would be 95.6% (153/160).
The final measure is the Precision, Accuracy, and
Control Test. In this test, the auditors look for locations or
items that may be suspect (open cartons or loose items).
The items are counted and compared with inventory. For
example, if 100 locations were tested and 5 locations were
incorrect, the PACT would be 95% (95/100).
It must be realized however, that cycle counting is
a measurement of inventory accuracy, not the cause. The
reason an inventory is accurate is that the procedures used
are disciplined and balanced, focusing on the fundamentals of best practice inventory management. This is true
for Supply Logistics as the cycle count results across all
warehouses and storerooms averaged 99.6% for 2014.
Submitted by:
Gary A. Smith, CFPIM, CSCP
Vice President, Division of Supply Logistics
New York City Transit
302
Chapter twelve
cause some possible cash flow issues for the supplier. Having their inventory in a customer facility does, however, tend to create a stronger relationship between the supplier
and the customer. Risk can also be reduced with strong consignment agreements, identifying such things as inventory levels and responsibility for lost or damaged inventory.
Another method being used in some companies is vendor-managed inventory
(VMI). VMI is actually an inventory management and planning system. Under a planning
system not using VMI, the customer’s planning system determines inventory need and the
inventory is acquired through the purchasing process. With VMI, the supplier assumes the
planning activities to determine what material is needed and when. Clearly, in order to
do this, there has to be a strong relationship between the supplier and customer, since the
way the supplier determines the customer material needs is by assuming the responsibility to have data on customer usage of the material. This implies that communication and
planning systems need to be acquired, at a cost, but the improvements in customer service, uncertainty of supply, fewer forecasting requirements, and often a reduction in total
inventory often makes up for the cost. The impact on storage should also be noted. An
additional benefit could be a closer, stronger relationship between customer and supplier.
technology aPPlications
Most imbalances in inventory records are caused by human error. Reading stock codes
and entering count quantities can be a source of many errors in any transaction, including
during the audit itself. Bar codes can reduce this error as they are machine-readable symbols and are widely used to gather information at all levels of retailing, distribution, and
manufacturing. The error rate for this method is extremely low compared to human error,
which is estimated to be as high as 3% for repetitive entries. Bar codes are standardized
by industry and are usually printed on a paper label or tag. Typically, they only contain
a unique identifier, such as part number, which can be referred to a database for further
information, such as price or description, as required. The automotive industry requires
labels designed to their specifications for layout and the type of code used. These labels
will include, in addition to the product code, the manufacturer, package quantity, date of
manufacture, and so forth.
Bar codes are read using a laser light, which picks up the reflection from the bars and
spaces on the label and is usually read from a short distance, although range is improving
with new designs. The use of bar codes improves the speed of data entry but, more importantly, improves the accuracy of the data retrieved.
Radio frequency identification (RFID) works in a way similar to bar codes, but
rather than light, it uses reflected radio waves from a small device or tag to receive its information. Unlike bar codes, RFID devices do not require a line of sight between the label and
the reader and can accurately identify products that are within containers or otherwise hidden from view. This feature makes RFID suited for use as a security device or under conditions where labels are hard to read or access. Railroads in North America use RFID tags on
all their rolling stock and locomotives. Trains are required to work in extreme conditions of
dirt and weather and the passive RFID tags located on each side of all vehicles allows the
accurate transfer of information such as owner, car number, and type of equipment. RFID
tags are more costly than printed bar codes, but the price is falling rapidly to only a few
cents per tag, encouraging their use in wider applications. Major retailers such as Wal-Mart
see the value in this method of gathering information and are demanding its use in many of
their products, further reducing costs throughout the supply chain.
Similar to bar codes, RFID tags provide identification numbers that can be used to
reference databases, but often have further information about the tagged item. In the railroad system, a car’s number can reference the car’s contents, destination, and so forth,
allowing delivery tracking and even safety/hazardous information access. Many organizations use RFID pass cards to access parking or secured areas. The user is identified by
the RFID signal and is allowed access accordingly. Management can then easily exercise
central control over these areas. When your season parking pass expires, access is automatically restricted.
Physical Inventory and Warehouse Management
303
Supply chains are driven by information, and the ability to retrieve timely, accurate,
and comprehensive information can lead to cost reductions. In previous chapters lead time
was shown to affect inventories and customer service levels. Bar code and RFID technologies can reduce lead time, allowing all members of a supply chain to reduce costs while at
the same time drastically reducing the number of occurrences of out of stock.
A warehouse management system (WMS) is a computer application that manages
all processes carried out by a warehouse, including receiving, picking, and shipping.
Output generated from ERP is executed by the WMS, such as the management and optimization of storage locations, cross-docking, cycle counting, automatic replenishment,
and the management of order picking and reverse logistics.
Inventory Traceability
It is often necessary or even required to be able to track material as it becomes a final
product and in the customer’s use. Sometimes it is discovered after a product has moved
through the supply chain that a problem exists with some material. It might be that the
material has a previously undetected quality problem or perhaps even might risk the
health or well-being of the customer (examples are food products, pharmaceuticals, and
automobiles). In such cases, it is important, for both cost and customer comfort, that the
specific products are located and recalled or repaired as quickly and efficiently as possible.
Batches of material are often given lot numbers, and the lot numbers can be included in the
bar codes. Standards have been developed (for example, a Global Trade Identification
Number, or GTIN) for tracking material, and RFID can also be used in certain cases.
suMMary
Previous chapters in this book have used theory and common sense to effectively manage inventories. This chapter looks at the physical handling and storage of inventory.
Warehousing is changing from a place to simply store material to a strategic activity
affecting a major investment to companies and a significant factor in improving customer
service. The basic warehouse activities of receiving, identifying, storing, retrieving, and
eventually shipping goods are all still required, but technology and the need for speed
in the supply chain are changing the way we do things. “A place for everything and
everything in its place” has given way to floating locations and point-of-use storage. As
movements are being sped up, there is an increasing need for control and security, and it
is important that workers know and understand proper procedures and the consequences
of errors. To provide high levels of customer service, inventory records must be accurate.
Cycle counting improves record accuracy but, more importantly, finds errors in the inventory systems so that processes can be continually improved. Bar code and RFID technologies are widely used in any modern inventory and warehousing system, improving data
accuracy and speeding the movement of product. Consignment inventory and VMI are
changing the replenishment of inventory and can impact issues of inventory storage.
Key terMs
Accessibility 291
Bar codes 302
Central storage 294
Count frequency 300
Consignment Inventory 301
Cube utilization 291
Cycle counting 299
Data governance 299
Fixed-location system 293
Global trade identification
number (GTIN) 303
Pallet positions 291
304
Chapter twelve
Periodic (annual) inventory 298
Point-of-use storage 293
Radio frequency identification (RFID) 302
Random-location system (Floatinglocation system) 293
Reserve stock 293
Tolerance 296
Vendor-managed inventory (VMI) 302
Warehouse management system
(WMS) 303
Working stock 292
Zone random storage 293
Questions
1. What are four objectives of warehouse operation?
2. Describe the eight warehouse activities as they would apply to a supermarket. Include in your
description where each activity takes place and who performs the activity.
3. What are cube utilization and accessibility?
4. Why is stock location important in a warehouse? Name and describe four basic systems of
stock location and give examples of each system from a retail setting.
5. Describe fixed and random systems for assigning locations to SKUs.
6. Name and describe three order-picking systems.
7. What is the difference between working stock and reserve stock?
8. What are the four steps in any transaction?
9. What are some of the results of poor inventory accuracy?
10. Six causes of poor inventory accuracy are discussed in the text. Name and describe each.
11. How should inventory accuracy be measured? What is tolerance? Why is it necessary?
12. What is the basis for setting tolerance?
13. What are the four major purposes of auditing inventory accuracy?
14. In taking a physical inventory, what are the three factors in preparation? Why is good preparation essential?
15. What are the four steps in taking a physical inventory?
16. Describe cycle counting. On what basis can the count frequency be determined?
17. Why is cycle counting a better way to audit inventory records than an annual physical inventory?
18. When are some good times to count inventory?
19. What information is typically stored in a bar code or RFID tag?
20. Give three examples where RFID would be preferred over a bar code.
21. How do bar codes and RFID reduce costs in the supply chain?
22. What is consignment inventory?
23. What is vendor-managed inventory?
24. How does vendor-managed inventory differ from consignment inventory?
ProbleMs
12.1. A company wants to store an SKU consisting of 5000 cartons on pallets, each containing 30 cartons. They are to be stored three high in the warehouse. How many
pallet positions are needed?
Answer.
56 pallet positions
12.2. A company has 6500 cartons to store on pallets. Each pallet takes 30 cartons, and the
cartons are stored four high. How many pallet positions are needed?
Physical Inventory and Warehouse Management
305
12.3. A company has an area for storing pallets as shown in the following diagram. How
many pallets measuring 48″ * 40″ can be stored three high if there is a 2″ space
between the pallets?
200'
4'
Aisle
4'
Answer.
342 pallets
12.4. A company has a warehouse with the dimensions shown in the following diagram.
How many pallets measuring 48″ * 40″ can be stored three high if there is to be a
2″ space between the pallets?
60'
4'
Aisle
4'
4'
Aisle
4'
12.5. A company wishes to store the following SKUs so there is 100% accessibility. The
items are stored on pallets that can be stacked three high.
a. How many pallet positions are needed?
b. What is the cube utilization?
c. If the company bought racking for storing the pallets, how many pallet positions
are needed to give 100% accessibility?
SKU
A
B
C
D
E
Total
Number of Pallets
13
4
10
13
14
Pallet Positions
Required
Answer. a. Pallet positions needed = 21
b. Cube utilization = 86%
c. 18 pallet positions
12.6. A company wants to store the following seven SKUs so there is 100% accessibility.
Items are stored on pallets that are stored four high.
a. How many pallet positions are needed?
b. What is the cube utilization?
306
Chapter twelve
c. If the company bought racking for storing the pallets, how many pallet positions
are needed to give 100% accessibility?
SKU
A
B
C
D
E
F
G
Total
Number of Pallets
14
17
40
33
55
22
34
Pallet Positions
Required
12.7. a. Which of the following items are within tolerance?
b. What is the percent accuracy by item?
Part
Number
Shelf
Count
Inventory
Record
A
650
635
+
– 3%
B
1205
1205
+
– 0%
C
1350
1500
+
– 5%
D
77
80
+
– 5%
E
38
40
+
– 3%
Difference
% Difference
Tolerance
Within
Tolerance?
Total
Answer. a. A, B, and D are within tolerance.
b. 60%
12.8. a. Which of the following items are within tolerance?
b. What is the percent accuracy by item?
Part
Number
Shelf
Count
Inventory
Record
A
75
80
+
– 3%
B
120
120
+
– 0%
C
1400
1500
+
– 5%
D
75
76
+
– 5%
E
68
66
+
– 2%
Total
Difference
% Difference
Tolerance
Within
Tolerance?
Physical Inventory and Warehouse Management
307
12.9. A company does an ABC analysis of its inventory and calculates that out of 5000
items, 22% can be classified as A items, 33% as B items, and the remainder as
C items. A decision is made that A items are to be cycle counted once a month,
B items every 3 months, and C items twice a year. Calculate the total counts and
the counts per day by classification. The company works 5 days per week and 50
weeks per year.
Classification
Number
of Items
Count
Frequency
per Year
Number
of Counts
per Year
% of Total
Counts
Counts
per Day
A
B
C
Total counts per year
Workdays per year
Total counts per day
Answer.
Counts per day A = 53; B = 26; C = 18
12.10. A company does an ABC analysis of its inventory and calculates that out of 10,000
items, 19% can be classified as A items, 30% as B items, and the remainder as C
items. A decision is made that A items are to be cycle counted twice a month, B
items every 3 months, and C items once a year. Calculate the total counts and the
counts per day by classification. There are 250 working days per year.
Classification
Number
of Items
A
B
C
Total counts per year
Workdays per year
Total counts per day
Count
Frequency
per Year
Number
of Counts
per Year
% of Total
Counts
Counts
per Day
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case study 12.1
CostMart Warehouse
Amy Gordon could not have been more pleased when she was first appointed as the new
inventory management supervisor for the CostMart regional warehouse. She had previously worked part time as a clerk in the local CostMart Department Store while she finished her university degree. After she got the degree, she was named as the section head
in charge of roughly one-fourth of the store. Now, a year later, she started to wonder about
that old adage, “Be careful what you ask for—you just might get it.”
Background
One constant problem Amy had complained about when she was head clerk was the
difficulties she had with the warehouse replenishing supplies for her areas of responsibility. She was sure the problem was not hers. The store used point-of-sale terminals, in
which the cash register doubled as a computer, instantly recognizing inventory movement. She also realized that shoplifting and other forms of loss were a constant problem
in retail stores, so she instructed all her clerks to spot count inventory in their areas
of responsibility whenever there was a lull in store traffic. The store computer had a
built-in program to suggest replenishment orders when the stock reduced to a certain
quantity. Amy had learned, of course, that these were only suggestions, since she knew
that some items were “faddish” and would have to be ordered sooner or not reordered
at all depending on how the fad was progressing. Some items were seasonal in nature,
which needed to be accommodated, and she was also aware when an item would go on
sale or have a special promotional campaign. These were announced well in advance
during the monthly managerial meetings, and she had good estimates as to the projected
impact on demand.
It was because she was so effective at managing the inventory in her area that she
was so vocal about the problems at the warehouse. It seemed that almost everything she
ordered for replenishment from the warehouse was a problem. Some items were late, occasionally by as much as six weeks. Other items were replenished in quantities far larger
or smaller than what was ordered, even if they were occasionally delivered on time. It
finally seemed to her that every warehouse delivery was a random event instead of the accurate filling of her orders. Her complaints to general management stemmed from the impact of the warehouse problems. Customers in her area were complaining more often and
louder as stockouts of various items became a pattern. Several customers had vowed to
never again shop at CostMart because of their frustration. One customer even physically
dragged Amy over to the sign above the entrance to the store—the one that proclaims
“CostMart—Where Customer Service Is in Charge”—and suggested that she could be
sued for false advertising.
In other cases, the quantity delivered was two to three times the amount she ordered.
She would often have to hold special unannounced sales to avoid being burdened with
the excessive inventory, especially since one of her performance metrics was inventory
dollars. Of course, one of the major performance metrics was profitability, and both the
stockouts and unannounced sales impacted that adversely. Finally, after one particularly
frustrating day, she told the general manager, “Maybe you should put me in charge of the
inventory over at the warehouse. I can control my own area here—I bet I could put that
place back in shape pretty fast!” Two weeks later, she was notified she was promoted to
inventory management supervisor for the warehouse.
the Current Situation
One of the first issues Amy faced was some not-so-subtle resentment from the warehouse
general supervisor, Henry “Hank” Anderson. Hank had been a supervisor for over 10
years, having worked his way up from an entry-level handler position. The inventory
supervisor position had been created specifically for Amy, as Hank had previously had
responsibility for the inventory. Their mutual boss had explained to Hank that the reduction in overall responsibility was not a demotion, but that growth in the warehouse made
Physical Inventory and Warehouse Management
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splitting the responsibilities a necessity. Although Hank outwardly acknowledged the
explanation, everyone knew that in reality he felt the change was a “slap in the face.”
That would normally be enough to cause some potential resentment, but in addition, as
Hank expressed in the lunchroom one day, “It’s not enough that they take some of my job
away, but then look who they give it to—a young, inexperienced college kid, and a female
at that! Everyone knows you can’t learn how to run a warehouse in some stupid college
classroom—you have to live it and breathe it to really understand it.”
Amy knew that the Hank situation was one she would have to work on, but in the
meantime she had to understand how things were run, and specifically why the warehouse
was causing all the problems she experienced at the store. Her first stop was to talk to Jane
Dawson, who was responsible for processing orders from the store. Jane explained the
situation from her perspective.
“I realize how much it must have bothered you to see how your store requests were
processed here, but it frustrates me too. I tried to group orders to prioritize due dates and
still have a full truckload to send to the store, but I was constantly having problems thrown
back at me. Sometimes I was told the warehouse couldn’t find the inventory. Other times
I was told that the quantity you ordered was less than a full box, and they couldn’t (or
wouldn’t) split the box up, so they were sending the full box. Then they would find something they couldn’t find when it was ordered a long time ago, so now that they found it
they were sending it. That order would, of course, take up so much room in the truck that
something else had to be left behind to be shipped later. Those problems, in combination
with true inventory shortages from supplier-missed shipments, always seem to put us behind and we are never able to ship what we are supposed to. None of this seemed to bother
Hank too much. Maybe you can do something to change the situation.”
Amy’s concern with what Jane told her was increased when she asked Jane if she
knew the accuracy of their inventory records and was told that she wasn’t sure, but the records were probably no more than 50% accurate. “How can that be?” Amy asked herself.
She knew they had recently installed a new computer system to handle the inventory, they
did cycle counting on a regular basis, and they used a “home base” storage system, where
each SKU had its own designated space in the warehouse racks. She realized she needed
to talk to one of the workers. She decided on Carl Carson, who had been with the company for about five years and had a reputation for being a dedicated and effective worker.
Amy told Carl what she already knew and asked him if he could provide any additional
information.
According to Carl, “What Jane told you is true, but what she didn’t tell you is that
a lot of it is her fault. If she would only give us some advance warning about what she
wants to send for the next shipment, we could probably do a better job of finding the material and staging it. What happens, though, is that she gives us this shipment list out of
the blue and expects us to find it all and get it ready in very little time. For one thing, she
doesn’t understand that it’s very impractical to break boxes apart in order to ship just the
quantity she wants. We don’t have a good way to package the partial box, and an open box
increases the chance for the remaining goods to be damaged or get dirty. Even if we had
a way to partially package, the time it would take would increase the chance we wouldn’t
make the shipment on time.
“Then there’s the problem of finding material. When supplier shipments come in,
they are often for more goods of a given SKU than we have room for on the rack. We put
the rest in an overflow area, but it’s really hard to keep track of. Even if we locate it in the
system correctly, someone will soon move it to get to something behind it. That person
will usually forget to record the move in the heat of getting a shipment ready. Since the
cycle counts don’t find it in the designated rack, the cycle counters adjust the count so the
system doesn’t even know it exists anymore. You might think we should expand the space
in the rack to hold the maximum amount of each SKU, but we would need a warehouse
at least double this size to do that—and there’s no way management would approve that.
I guess the only good thing about the situation is that when we do find some lost material
that was requested earlier, we ship it to make up for not shipping it earlier.”
Amy was beginning to feel a tightening in her stomach as she realized the extent of
the problem here. She almost had to force herself to talk to Crista Chávez, who worked for
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the purchasing department and was responsible for warehouse ordering. Crista was also
considered to be experienced, capable, and dedicated to doing a good job for the company.
Crista added the following perspective:
“We have good suppliers, but they’re not miracle workers. Since we beat them up so
badly on price most of the time, I can understand why they’re not interested in doing more
than they already are. The problem is we can’t seem to get our own house in order to give
them a good idea of what we need, and when we really need it. To do that, we would need
to know what the warehouse needs and when, and also the existing inventory of the item.
We seem to have no idea what we need, and the inventory records are a joke. I spend most
of my day changing order dates, order quantities, or expediting orders to fill a shortage,
and often the shortage isn’t really a shortage at all. Our only hope has been to order early
and increase our order quantities to ensure we have enough safety stock to cover the inventory accuracy problems. I’ve complained to Hank several times, but all he says is that
it’s my job to pull the suppliers in line, that the problem is obviously theirs.”
At least by this point, Amy had a better perspective about the problems. Unfortunately,
it was now up to her to fix them. She wished she had never opened her mouth to complain
about the problems. Too late for that—she now had to develop a strategy to deal with what
she had been handed.
assignment
1. Structure what you think the problems are. Be sure to separate the problems from the
symptoms.
2. Assume Amy needs to build a data-based case to convince her boss and start to “win
over” Hank. What data should she gather to help her build the case?
3. Develop a model of how you think the warehouse should work in this environment.
4. Develop a time-phased plan to move from the present situation to the model you
developed in question 3.
Chapter
thirteen
Physical Distribution
introDuction
In Chapter 1 it was pointed out that a supply chain is comprised of a series of suppliers
and customers linked together by a physical distribution system. Usually the supply chain
consists of several companies linked in this way. This chapter will discuss the channels of
physical distribution, which cover the physical movement of goods as well as the change
of ownership that occurs throughout the supply chain. Physical distribution involves the
movement of goods through the various transportation modes, the inventories that exist in
transit and in distribution centers and production facilities, the physical handling of goods,
and the need for protective packaging. Multiple warehouse decisions involve the costs
associated with adding more warehouses and the effect on customer service.
Physical distribution is the movement of materials from the producer to the consumer,
and is the responsibility of the distribution or logistics department. Figure 13.1 shows the
relationship of the various functions in this type of system.
In Figure 13.1, the movement of materials is divided into two functions: physical
supply and physical distribution. Physical supply is the movement and storage of goods
(incoming items) from suppliers to manufacturing. Depending on the conditions of sale,
the cost may be paid by either the supplier or the customer, but it is ultimately passed on
to the customer. Physical distribution, on the other hand, is the movement and storage of
finished goods from the end of production to the customer (outgoing items). The particular path in which the goods move, through distribution centers, wholesalers, and retailers,
is called the channel of distribution.
Channels of Distribution
A channel of distribution is one or more companies or individuals who participate in the
flow of goods and/or services from the producer to the final user or consumer. Sometimes
a company delivers directly to its customers, but often it uses other companies or
S
U
P
P
L
I
E
R
Physical
Supply
MANUFACTURER
DISTRIBUTION
SYSTEM
Manufacturing
Planning and
Control
Physical Distribution
C
U
S
T
O
M
E
R
DOMINANT FLOW OF PRODUCTS AND SERVICES
DOMINANT FLOW OF DEMAND AND DESIGN INFORMATION
Figure 13.1 Supply chain (logistics system).
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DISTRIBUTION
CHANNEL
TRANSACTION
CHANNEL
FACTORY
WAREHOUSE
GENERAL SALES
OFFICE
REGIONAL
WAREHOUSE
DISTRICT SALES
OFFICE
PUBLIC
WAREHOUSE
DISTRIBUTOR
LOCAL
DELIVERY
RETAILER
COMPANY
TRUCK
COMMON
CARRIER
CONSUMER
Figure 13.2 Separation of distribution and transaction channels.
individuals to distribute some or all of its products to the final consumer. These companies or individuals are called intermediaries. Examples of intermediaries are wholesalers,
agents, transportation companies, and warehouses.
There are two related channels involved: transaction channel and distribution channel. The transaction channel is concerned with the transfer of ownership and funds. Its
function is to negotiate, contract, and sell. The distribution channel is concerned with the
transfer or physical delivery of the goods or services. The same intermediary may perform
both functions, but not necessarily.
Figure 13.2 shows an example of the separation of distribution and transaction channels. An example would be a company distributing a major appliance such as a refrigerator or stove. In such a process, the retailer usually carries only display models. When the
customer orders an appliance, delivery is made from either the regional warehouse or the
public warehouse.
Although it can be argued that one firm’s physical supply is another firm’s physical
distribution, frequently there are important differences, particularly as they relate to the
bulk and physical condition of raw materials and finished goods. The logistics problems
that occur in moving and storing iron ore are quite different from those that occur with
sheet steel. These differences influence the design of a logistics system and are important
in deciding the location of distribution centers and factories. This text refers to both physical distribution and physical supply as physical distribution, but the differences for any
particular company should be remembered.
Physical distribution is vital in everyday life. Usually, manufacturers, customers,
and potential customers are widely dispersed geographically. If manufacturers serve
only their local market, they restrict their potential for growth and profit. By extending
its market, a firm can gain economies of scale in manufacturing, reduce the cost of purchases by volume discounts, and improve its profitability. However, to extend markets
requires a larger and well-run distribution process. Manufacturing adds value to a product by taking the raw materials and creating something more useful. Bread is made from
grain and is far more useful to humans than the grain itself. Distribution adds value by
placing goods in markets where they are available to the consumer at the time the consumer wants them.
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313
The specific way in which materials move depends upon many factors. For example:
The number and types of channels of distribution that the firm is using. How
many intermediaries the organization is going through to travel from producer to consumer, such as producer to wholesaler to retailer to consumer.
■■ The types of markets served. Market characteristics such as the geographic dispersion of the market, the number of customers, and the size of orders.
■■ The characteristics of the product. Factors such as weight, density, fragility, and
perishability, and other special handling and storage considerations.
■■ The type of transportation available to move the material. Modes of transportation
such as trains, ships, planes, and trucks.
■■
All of these factors are closely related. For instance, florists selling a perishable product
to a local market will sell directly and probably use their own trucks. However, a national
canning company selling a nonperishable product to a national market through a distribution channel composed of wholesalers and retailers may use trucks and rail transport.
Reverse Logistics
As discussed in Chapter 2, reverse logistics is becoming increasingly important in the
management of a supply chain because of the growth of online shopping and the desire
to reuse and recycle materials once a customer is finished with the product. Companies
must deal with the flow of goods coming back from the final customer or other companies
in the distribution channel. This reverse supply chain may be caused by several factors,
including the following:
Quality issues in the goods resulting in customer returns, warranty replacements, and
so forth.
■■ Financial pressures on distributors to reduce slow-moving or unwanted inventories.
■■ The return of reusable materials, such as packaging, after delivery.
■■ The return of components and materials for recycling or disposal, such as electronics.
■■
APICS Dictionary, 14th edition, defines reverse logistics as a “complete supply chain
dedicated to the reverse flow of products and materials for the purpose of returns, repair,
remanufacture, and/or recycling.”
In some distribution channels reverse logistics can represent major costs, which are
growing partly due to the increased use of the internet. Sales through the internet tend to
be to a wider geographic area and may have a higher-than-normal frequency of returns.
The total costs associated with reverse logistics are estimated to exceed $50 billion per
year in the United States alone. The amount of goods returned in the publishing industry
can be as high as 50% of the original shipments as magazines go out of date, or 90% in
the automotive parts industry for the rebuilding of starter motors and alternators. In addition, companies are being forced to take responsibility for the return of packaging. There
are two main categories of reverse logistics: asset recovery, which is the return of actual
products, and green reverse logistics, which represents the responsibility of the supplier
to dispose of packaging materials or environmentally sensitive materials such as heavy
metals and other restricted materials.
The costs of green reverse logistics are reduced through the use of reusable packaging, such as bins or racks rather than corrugated containers, or an overall reduction in the
amount of packaging. Environmentally sensitive materials can be sorted and either reused
in manufacturing or disposed of in the most cost-effective method possible, hopefully
avoiding landfills. Refillable beverage containers reduce the need for landfill space but
impose a cost on the producer for sorting and handling.
Reducing costs associated with asset recovery involves coordinating the handling of
the materials, perhaps with the outbound flow of new goods. Information on the returned
materials is necessary to ensure proper reuse or disposal of the materials. If the return
will generate a credit to the sender, this will also require information. An example of this
working effectively occurs with the replacement of a starter motor in a car. The installer
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orders a starter motor and installs it. The old starter motor, which should be the same size
and model as the replacement, is placed in the original carton and sent back to the supplier
for a credit. The carton will have all the information describing the motor and can be used
for identification as it travels back to the rebuilder. To reduce costs, distributors will coordinate the outbound shipments of new motors with the return of old motors for rebuilding.
Assets are returned for many reasons, which can include the following:
Quality demands by final customers (both real and perceived).
Damaged or defective products.
■■ Inventories that result from over-forecast demand.
■■ Seasonal inventories.
■■ Out-of-date inventories.
■■ Remanufacturing and refurbishment of products.
■■
■■
Returned goods can be:
Returned to inventory.
Refurbished for resale.
■■ Sold into alternate markets.
■■ Broken down into reusable components.
■■ Sorted to recover valuable materials (further reducing disposal costs).
■■
■■
If the distribution channel is very simple, as in the return of starter motors example,
organizations can reduce reverse logistics costs by coordinating outbound and inbound
shipments. Should the distribution channel, however, be very complex, third-party logistics
companies (3PLs), discussed later in this chapter, can be used to centralize the handling and
disposition of the returned goods. The 3PLs are also very good at providing the information
to track the flow of the goods. Companies that take a strategic approach to reverse logistics
can recover significant costs and provide better inventory levels throughout the supply
chain. These companies will also build reputations as good corporate citizens.
Physical Distribution
Physical distribution is responsible for delivering to the customer what is wanted on time
and at minimum cost. The objective of distribution management is to design and operate a
distribution channel that attains the required level of customer service and does so at least
cost. To reach this objective, all activities involved in the movement and storage of goods
must be organized into an integrated system.
Activities in Physical Distribution
A physical distribution system involves six interrelated activities that affect customer service and the cost of providing it:
1. Transportation. Transportation involves the various methods of moving goods
outside the firm’s buildings. For most firms, transportation is the single highest cost
in distribution, usually accounting for 30 to 60% of distribution costs. Transportation
only adds value to the product if the customer is willing to pay for the inventory to be
located closer to them.
2. Distribution inventory. Distribution inventory includes all finished goods inventory
at any point in the distribution channel. In cost terms, it is the second most important
item in distribution, accounting for about 25 to 30% of the cost of distribution. Again,
if the customer is willing to pay for it, inventories can create value by placing the
product close to the customer.
3. Warehouses (distribution centers). Warehouses are used to store inventory. The
management of warehouses makes decisions on site selection, number of distribution
centers in the system, layout, and methods of receiving, storing, and retrieving goods.
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315
4. Material handling. Material handling is the movement and storage of goods inside
the distribution center. The type of material handling equipment used affects the efficiency and cost of operating the distribution center. Material handling equipment represents a capital cost, and a trade-off exists between this capital cost and the operating
costs of the distribution center. The labor associated with material handling represents
a cost of carrying inventory.
5. Protective packaging. Goods moving in a distribution channel must be contained,
protected, and identified. In addition, goods are moved and stored in packages and
must fit into the dimension of the storage spaces and the transportation vehicles.
6. Order processing and communication. Order processing includes all activities
needed to fill customer orders. Order processing represents a time element in delivery
and is an important part of customer service. Many intermediaries are involved in the
movement of goods, and good communication is essential to a successful distribution
system.
Total Cost Concept
As mentioned, the objective of distribution management is to provide the required level
of customer service at the least total cost. This does not mean that transportation costs or
inventory costs or any one activity cost should be a minimum but that the total of all costs
should be a minimum. What happens to one activity has an effect on other activities, total
cost, and the service level. Management must treat the process as a whole and understand
the relationships among the activities.
example Problem
A company normally ships a product by rail. Transport by rail costs $200, and the
transit time is 10 days. However, the goods can be moved by air at a cost of $1000
and will take 1 day to deliver. The cost of inventory in transit for a particular shipment
is $100 per day. What are the costs involved in their decision?
Answer
Rail
Air
Transportation Cost
$ 200
$1000
In-Transit Inventory Carrying Cost
Total
1000
$1200
100
$1100
There are two related principles illustrated here:
1. Cost trade-off. The cost of transportation increased with the use of air transport,
but the cost of carrying inventory decreased. There was a cost trade-off between
the two.
2. Total cost. By considering all of the costs and not just any one cost, the total cost
is reduced. Note also that even though no cost is attributed to it, customer service is
improved by reducing the transit time. The total cost should also reflect the effect of
the decision on other departments, such as production and marketing.
The preceding example does not mean that using faster transport always results in
savings. For example, if the goods being moved are of low value and inventory carrying cost is only $10 per day, rail will be cheaper. In addition, other costs may have to be
considered.
Most of the decisions in distribution, and indeed much of what is done in business
and in our own lives, involve trade-offs and an appreciation of the total costs involved. In
this section, the emphasis is on the costs and trade-offs incurred and on improvement in
customer service. Generally, but not always, an increase in customer service requires an
increase in cost, which is one of the major trade-offs.
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Global Distribution
Global distribution is the movement of goods to and from locations around the world.
Organizations are moving toward the global sourcing and selling of goods due to lower
manufacturing costs in other nations and the ability of both foreign and domestic manufacturers to supply a global market. Global distribution of goods is similar to movement
within one particular continent since information is needed to control inventories, customer needs must be satisfied, and carriers depend on communications to reach their destinations. Some differences, however, must be taken into consideration when dealing with
organizations around the world: distance, language, culture, currency, and measurement.
The longer the distance that goods must travel, the greater the time to reach markets.
Such distances may require conducting business across different time zones. As the goods
cross borders, the languages spoken may differ for the manufacturer, warehouse agencies, and carriers. Cultural differences can include the methods of conducting business,
the occurrence of religious holidays, and the local work ethic. Currency exchange and
international fund transfers are becoming increasingly easy; however, fluctuations in currencies can change costs dramatically and must be taken into account when assessing the
risks involved in deciding where to source goods. Measurement systems will differ around
the world, and a good example of this is weight. A ton is 2000 pounds, a long ton is 2240
pounds, and a metric ton is 2205 pounds. The units of measure for weight can depend on
the country that is handling the goods. Overall, global distribution can be very complicated for individuals and companies facing this challenge.
Fortunately, technologies and supply chain management processes in place now can
alleviate some of these problems. Time is less of an issue. The internet allows companies
to conduct business at all hours of the day, with fewer misinterpretations of order information such as sizes, quantities, and descriptions. International standards help solve many
distribution issues. The International Organization for Standardization (ISO) (discussed
in Chapter 16), for example, has established standards for ocean shipping containers that
allow the seamless handling of goods from ship, to rail, to truck across virtually all nations.
Some standards have a long history. As early as 1936, Incoterms were developed by the
Paris-based International Chamber of Commerce to provide internationally accepted regulations for trade terms such as export packing costs, customs clearance, inland and ocean
transportation costs, and damage insurance. Incoterms are explained later in this chapter.
Global distribution will continue to grow, become easier, and increasingly allow
companies to manufacture goods at competitive rates and to sell their products in a global
market. As global distribution becomes essential, people working in distribution will need
to expand their knowledge of global-based supply systems and international business
practices to remain effective.
3PLs: Third Party Logistics Providers
Buyers and suppliers of goods often work with a third party who is in a position to offer
physical distribution services at less cost than the buyer or seller would incur performing
these services themselves. Beyond providing delivery services, a third party logistic provider (3PL) may supply services such as warehousing, electronic data interchange (EDI),
packaging, warehousing, freight forwarding, order processing, product tracking, and
delivery. They can provide these services at an economical rate since they already have
the infrastructure in place and combine one company’s distribution needs with those of
their other clients. FedEx is an example of this, providing customers over geographically
dispersed markets with local warehousing, inventory management, labeling requirements,
and more.
3PLs allow businesses to concentrate on their core competencies and leave the problems associated with delivering or servicing their products to someone else. They also
react well to problems of scale where the volumes shipped or the number of customers
either increase or decrease. 3PLs are especially effective in emerging markets where a
supplier needs the capabilities of a local warehouse but does not have the resources or
volumes to justify their own facility.
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4PLs: Fourth Party Logistics Providers
A supply chain partner similar to a 3PL is a fourth party logistics provider (4PL). The
4PL is a logistics specialist who manages the entire logistics operation for the customer. It
includes those activities mentioned under 3PL, but also includes the possible subcontracting of other parties to perform some of the services, coordinating the logistics efforts of all
parties. According to APICS Dictionary, 14th edition, it differs from 3PL as it serves as
“… an interface between the client and multiple logistics providers.”
Physical Distribution interfaces
By taking the goods produced by manufacturing and delivering them to the customer,
physical distribution provides a bridge between marketing and production. As such,
there are several important interfaces among physical distribution and production and
marketing.
Marketing
Although physical distribution interacts with all departments in a business, its closest
relationship is probably with marketing. The “marketing mix” is made up of product,
promotion, price, and place, and the latter is created by physical distribution. Marketing
is responsible for creating demand. This is accomplished by such methods as personal
selling, advertising, sales promotion, merchandising, and pricing. Physical distribution
is responsible for giving the customer possession of the goods and does so by operating
distribution centers, transportation channels, inventories, and order processing. It has the
responsibility of meeting the customer service levels established by marketing and the
senior management of the firm.
Physical distribution contributes to creating demand. Prompt delivery, product availability, and accurate order filling are important competitive tools in promoting a firm’s
products. The distribution system is a cost, so its efficiency and effectiveness influence the
company’s ability to price competitively. All of these affect company profits.
Production
Physical supply establishes the flow of material into the production process. The component and raw material service level must usually be very high because the cost of interrupted production schedules caused by raw material shortages is usually enormous.
There are many factors involved in selecting a site for a factory, but an important
one is the cost and availability of transportation for raw materials to the factory and the
movement of finished goods to the marketplace. Sometimes the location of factories is
decided largely by the sources and transportation links of raw materials. This is particularly true where the raw materials are bulky and of relatively low value compared to the
finished product. The location of steel mills on the Great Lakes of the United States is a
good example. The basic raw material, iron ore, is bulky, heavy, and of low unit value.
Transportation costs must be kept low to make a steel mill profitable. Iron ore from mines
in either northern Quebec, Canada, or the state of Minnesota is transported to the mills by
cargo ship, which is typically the least costly mode of transportation. In other cases, the
availability of low-cost transportation makes it possible to locate in areas remote from
markets, but where labor is inexpensive.
Unless a firm is delivering finished goods directly to a customer, demand on the factory is created by the distribution center orders and not directly by the final customer. As
noted in Chapter 11, this can have severe implications on the demand pattern at the factory. Although the demand from customers may be relatively uniform, the factory reacts
to the demand from the distribution centers for replenishment stock. If the distribution
centers are using an order point technique, the demand on the factory will not be uniform
and will be dependent rather than independent. The distribution channel is the factory’s
customer, and the way that distribution interfaces with the factory will influence the efficiency of factory operations.
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Chapter thirteen
transPortation
Transportation is an essential ingredient in the economic development of any area. It
brings together raw materials for production of marketable commodities and distributes
the products of industry to the marketplace. As such, it is a major contributor to the economic and social fabric of a society and aids economic development of regional areas.
The carriers of transportation can be divided into five basic modes:
1. Rail.
2. Road, including trucks, buses, and automobiles.
3. Air.
4. Water, including oceangoing, inland, and coastal ships.
5. Pipeline.
Each mode has different cost and service characteristics. These determine which
method is appropriate for the types of goods to be moved. Certain types of traffic are simply more logically moved with one mode than they are with another. For example, trucks
are best suited to moving small quantities to widely dispersed markets, but trains are best
suited to moving large quantities of bulky cargo, such as grain.
Costs of Carriage
To provide transportation service, any carrier, whatever mode, must have certain basic
physical elements. These elements are ways, terminals, and vehicles. Each results in a cost
to the carrier and, depending on the mode and the carrier, may be either capital (fixed) or
operating (variable) costs. Fixed costs are costs that do not change with the volume of
goods carried. The purchase cost of a truck owned by the carrier is a fixed cost. No matter how much it is used, the cost of the vehicle does not change. However, many costs of
operation, such as fuel, maintenance, and driver’s wages, depend on the use made of the
truck. These are variable costs.
Ways are the paths over which the carrier operates. They include the right-of-way
(land area being used), plus any road, tracks, or other physical facilities needed on the
right-of-way. The nature of the way and how it is paid for vary with the mode. Ways may
be owned and operated by the government, by the carrier, or provided by nature.
Terminals are places where carriers load and unload goods to and from vehicles and
make connections between local pickup and delivery service and line haul service. Other
functions performed at terminals are: weighing; connecting with other routes and carriers;
vehicle routing, dispatching, and maintenance; and administration and paperwork. The
nature, size, and complexity of the terminal varies with the mode and size of the firm and
the types of goods carried. Terminals are generally owned and operated by the carrier but,
in some special circumstances, may be publicly owned and operated.
Vehicles of various types are used in all modes except pipelines. They serve as carrying and power units to move the goods over the ways. The carrier usually owns or leases
the vehicles, although sometimes the shipper owns or leases them.
Besides ways, terminals, and vehicles, a carrier will have other costs such as maintenance, labor, fuel, and administration. These are generally part of operating costs and may
be fixed or variable.
Rail
Railways provide their own ways, terminals, and vehicles, all of which represent a large
capital investment. This means that most of the total cost of operating a railway is fixed.
Thus, railways must have a high volume of traffic to absorb the fixed costs. They will
not want to install and operate rail lines unless there is a large enough volume of traffic.
Trains move goods by trainloads composed of perhaps a hundred cars, each with a carrying capacity of around 160,000 pounds.
Therefore, railways are best able to move large volumes of bulky goods over long
distances. Their frequency of departure will be less than trucks, which can move when one
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319
truck is loaded. Rail speed is good over long distances, the service is generally reliable,
and trains are flexible about the goods they can carry. Train service is cheaper than road
service for large quantities of bulky commodities, such as coal, grain, potash, and containers moved over long distances.
Road
Trucks do not provide their own ways (roads and highways) but pay a fee to the government
in the form of licenses, gasoline, and other taxes and tolls for the use of roads. Terminals are
usually owned and operated by the carrier but may be either privately owned or owned by
the government. Vehicles are owned, or leased, and operated by the carrier. If owned, they
are a major capital expense. However, in comparison to other modes, the cost of a vehicle is
small. This means that for road carriers, most of their costs are operating (variable) in nature.
Trucks can provide door-to-door service as long as there is a suitable surface on
which to drive. In the United States and Canada, the road network is comprehensive. The
unit of movement is a truckload, which can be up to about 100,000 pounds. These two
factors—the excellent road system and the relatively small unit of movement—mean that
trucks can provide fast flexible service almost anywhere in North America. Trucks are
particularly suited to distribution of relatively small-volume goods to a dispersed market.
Air
Air transport does not have ways in the sense of fixed physical roadbeds, but it does
require an airway system that includes air traffic control and navigation systems. These
are usually provided by the government. Carriers pay a user charge that is a variable cost
to them. Terminals include all of the airport facilities, most of which are provided by a
local government. However, carriers are usually responsible for providing their own cargo
terminals and maintenance facilities, either by owning or renting the space. The carrier
provides the aircraft through either ownership or leasing. The aircraft are expensive and
are the single most important cost element for the airline. Since operating costs are high,
airlines’ costs are mainly variable.
The main advantage of air transport is speed of service, especially over long distances.
Some cargo travels in passenger aircraft, and thus many delivery schedules are tied to those
of passenger service. The service destination is flexible provided there is a suitable landing
strip. Transportation cost for air cargo is higher than for other modes. For these reasons, air
transport is most often suitable for high-value, low-weight cargo or for emergency items.
Water
Waterways are provided by nature or by nature with the assistance of the government. The
St. Lawrence Seaway system is an example of this. The carrier thus has no capital cost in
providing the ways but may have to pay a fee for using the waterway.
Terminals may be provided by the government but are increasingly privately owned.
In either case, the carrier will pay a fee to use them. Thus, terminals are mainly a variable
cost. Vehicles (ships) are either owned or leased by the carrier and represent the major
capital or fixed cost to the carrier.
The main advantage of water transport is cost. Operating costs are low, and since the
ships have a relatively large capacity, the fixed costs can be absorbed over large volumes.
Ships are slow and operate door to door only if the shipper and the consignee are on a
waterway. Therefore, water transportation is most useful for moving low-value, bulky
cargo over relatively long distances where waterways are available.
Pipelines
Pipelines are unique among the modes of transportation in that they move only gas, oil,
and refined products on a widespread basis. As such, they are of little interest to most
users of transportation. Capital costs for ways and pipelines are high and are borne by the
carrier, but operating costs are very low.
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Intermodal
When deliveries cannot be accomplished using a single method of transportation, multiple
methods can be combined into a hybrid mode known as intermodal. Product travels part
of the way on one mode, and is then transferred to another. Examples of this include:
Piggyback, referring to placing a truck trailer or container on the back of a railcar.
Fishyback, referring to truck trailers or containers that are transported partway by ship
or barge.
■■ Birdyback, referring to cargo that is shipped using both road and air shipments.
■■
■■
Transportation Scheduling
Whichever mode of transportation is selected, the scheduling of shipments is a critical
part of physical distribution. Shipments must be planned to optimize the transportation
network, taking speed and cost into consideration, as well as the customer’s needs. Routes
must be selected, and may include the ability to perform backhauling, which refers to
transporting a full or partial load on the return trip from the destination point to the point
of origin. Since the bulk of a logistics budget may be used for transportation costs, constraints must be managed to ensure that goods are delivered on a timely and cost-effective
basis. Some constraints are inherently based on the type of carrier selected, such as access
to ports or railroads, air freight schedules, and so forth. Other constraints may occur
unplanned, such as worker strikes, natural disasters that limit or prohibit accessibility and
the movement of goods, and terrorism.
Transportation management systems (TMS) can be used to automate the transportation planning and scheduling process and deal with any bottlenecks that occur. These
applications automate the planning and decision making for activities such as determining the most efficient routes, carrier rate acceptance, dispatching, tracing shipments,
fleet management, and generating required documents for domestic and import/export
shipments. The TMS load planning/building process determines the appropriate mode of
transportation and optimizes the shipment based on volume, density, and cost. For example, it can provide information required to make a decision on whether a shipment should
be shipped LTL, or held for additional goods for a truckload shipment.
legal tyPes of carriage
Carriers are legally classified as public (for hire) or private (not for hire). In the latter,
individuals or firms own or lease their vehicles and use them to move their own goods.
Public transport, on the other hand, is in the business of hauling for others for pay. All
modes of transport have public and for-hire carriers.
For-hire carriers are subject to economic regulation by federal, state, or municipal
governments. Depending on the jurisdiction, economic regulation may be more or less
severe, and in recent years, there has been a strong move by government to reduce regulations. Economic regulation has centered on three areas:
1. Regulation of rates.
2. Control of routes and service levels.
3. Control of market entry and exit.
Private carriers are not subject to economic regulation but, like public carriers, are
regulated in such matters as public safety, license fees, and taxes.
For Hire
A for-hire carrier may carry goods for the public as a common carrier or under contract to
a specified shipper.
Common carriers make a standing offer to serve the public. This means that whatever products they offer to carry will be carried for anyone wanting their service. With
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some minor exceptions, they can carry only those commodities they are licensed to carry.
For instance, a household mover cannot carry gravel or fresh vegetables. Common carriers
provide the following:
Service available to the public.
Service to designated points or in designated areas.
■■ Scheduled service.
■■ Service of a given class of movement or commodity.
■■
■■
Contract carriers haul only for those with whom they have a specific formal contract of service, not the general public. Contract carriers offer a service according to a
contractual agreement signed with a specific shipper. The contract specifies the character
of the service, performance, and charges.
Private
Private carriers own or lease their equipment and operate it themselves. This means
investment in equipment, insurance, and maintenance expense. A company normally considers operating its own fleet only if the volume of transport is high enough to justify the
capital expense.
Service Capability
Service capability depends on the availability of transportation service, which in turn
depends on the control that the shipper has over the transportation agency. The shipper
must go to the marketplace to hire a common carrier and is subject to the schedules and
regulations of that carrier. Least control is exercised over common carriers. Shippers can
exercise most control over their own vehicles and have the highest service capability with
private carriage.
Other Transportation Agencies
There are several transportation agencies that use the various modes or combinations of
the modes. Some of these are the post office, freight forwarders, couriers, and shippers.
They all provide a transportation service, usually as a common carrier. They may own the
vehicles, or they may contract with carriers to move their goods. Usually, they consolidate
small shipments into large shipments to make economic loads.
transPortation cost elements
There are four basic cost elements in transportation. Knowledge of these costs enables a
shipper to get a better price by selecting the right shipping mode. The four basic costs are
as follows:
1. Line haul.
2. Pickup and delivery.
3. Terminal-handling.
4. Billing and collecting.
Motor transport will be used as an example, but the principles are the same for all
modes.
Goods move either directly from the shipper to the consignee or through a terminal.
In the latter, they are picked up in some vehicle suitable for short-haul local travel. They
are then delivered to a terminal where they are sorted according to destination and loaded
onto highway vehicles for travel to a destination terminal. There they are again sorted,
loaded on local delivery trucks, and taken to the consignee. Figure 13.3 shows this pattern
schematically.
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FULL LOAD
SHIPMENTS
S
H
I
P
P
E
R
Local
T
E
R
M
I
N
A
L
Full Load
Long Distance
T
E
R
M
I
N
A
L
Local
C
O
N
S
I
G
N
E
E
Figure 13.3 Shipping patterns.
Line Haul Costs
When goods are shipped, they are sent in a moving container that has a weight and volume
capacity. The carrier, private or for hire, has basic costs to move this container, which
exist whether the container is full or not. These are called line haul costs. For a truck,
these include such items as the driver’s wages and depreciation due to usage. These costs
vary with the distance traveled, not the weight carried. The carrier has essentially the same
basic costs whether the truck moves full or empty. If it is half full, the basic costs must be
spread over only those goods in the truck.
Therefore, the total line haul cost varies directly with the cost per mile and the distance shipped, not on the weight shipped. For example, if for a given product the line haul
cost is $3 per mile and the distance is 100 miles, the total line haul cost is $300. If the shipper sends 50,000 pounds, the total line haul cost is the same as if 10,000 pounds is sent.
Even though weight is not a factor in line haul costs, it can be used to designate the
cost to the shipper. The unit of measure used for weight and mass may vary according to
country, and includes ton, metric ton, pound, kilogram, and hundredweight (cwt.). The
following example uses a calculation of line haul cost per hundred weight, which is a
common designation for shipping commodities within North America.
300
500
= $0.60 per cwt. 3for 50,000 lbs. 1500 cwt.24
LHC>cwt. =
300
100
= $3 per cwt. 3for 10,000 lbs. 1100 cwt.24
LHC>cwt. =
Weight can also be used to compare line haul costs by determining the cost per a certain weight.
example Problem
For a particular commodity, the line haul cost is $2.50 per mile. For a trip of 500 miles
and a shipment of 6.5 tons, what is the cost of shipping per ton? If the shipment is
increased to 10 tons, what is the saving in cost per ton?
Answer
Total line haul cost = $2 .5 * 500 = $1250
Cost per ton = $1250 , 6 .5 = $192 .31
If shipping 10 tons:
Cost per ton = $1250 , 10 = $125 .00
Savings per ton = $192 .31 - $125.00 = $67.31
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Volume
The carrier has two limitations or capacity restrictions on how much can be moved on any
one trip: the weight limitation and the cubic volume limitation of the vehicle. With some
commodities, their density is such that the volume limitation is reached before the weight
limitation. If the shipper wants to ship more, a method of increasing the density of the
goods must be found. This is one reason that some lightweight products are made so they
nest (e.g., disposable cups) and bicycles and wheelbarrows are shipped in an unassembled
state. This is not to frustrate the poor mortals who try to assemble them but to increase the
density of the product so more weight can be shipped in a given vehicle. The same principle applies to goods stored in distribution centers. The more compact they are, the more
can be stored in a given space. Therefore, if shippers want to reduce transportation cost,
they should (a) increase the weight shipped and (b) maximize density.
example Problem
A company ships barbecues fully assembled. The average line haul cost per shipment
is $12.50 per mile, and the truck carries 100 assembled barbecues. The company
decides to ship the barbecues unassembled and figures it can ship 500 barbecues in
a truck. Calculate the line haul cost per barbecue assembled and unassembled. If the
average trip is 300 miles, calculate the saving per barbecue.
Answer
Line@haul cost assembled = $12.50 , 100 = $0.125 per barbecue per mile
Line@haul cost unassembled = $12.50 , 500 = $0.025 per barbecue per mile
PhysicAl Distribution
Saving per mile = $0.125 - 0.025 = $0.10
Trip saving = 300 * $0.10 = $30.00 per barbecue
Pickup and Delivery Costs
Pickup and delivery costs are similar to line haul costs except that the cost depends more
on the time spent than on the distance traveled. The carrier will charge for each pickup and
the weight picked up. If a shipper is making several shipments, it will be less expensive if
they are consolidated and picked up on one trip.
Terminal-Handling Costs
Terminal-handling costs depend on the number of times a shipment must be loaded,
handled, and unloaded. If full truckloads are shipped, the goods do not need to be handled
in the terminal but can go directly to the consignee. If partial loads are shipped, they must
be taken to the terminal, unloaded, sorted, and loaded onto a highway vehicle. At the
destination, the goods must be unloaded, sorted, and loaded onto a local delivery vehicle.
The process of receiving includes not only the physical receipt of the material, but
may also encompass inspection of the shipment to make sure it complies with the quantity
on the purchase order, as well as any damage or other quality issues. Bulk packages may
need to be broken down for storage or packaged for delivery.
Preparing an order for shipping from the warehouse typically involves picking the
required quantities from warehouse storage, performing any necessary repackaging, labeling the package with any required delivery and safety information, and then loading it
onto the carrier.
From the time a product is received until it is reshipped, an individual parcel may be
handled multiple times. A shipper who has many customers, each ordering small quantities, will expect the terminal-handling costs to be high because there will be a handling
charge for each package.
A basic rule for reducing terminal-handling costs is to reduce handling effort by
consolidating shipments into fewer parcels. In addition, technology can help to lower the
Chapter thirteen
time it takes to perform these activities. Barcoding and RFID technology can simplify the
process, as well as automated guided vehicle systems, automated storage and retrieval systems (AS/RS), and robotics. Consideration must be given to the changes in packaging and
installation of technology required to utilize the tools.
Billing and Collecting Costs
Every time a shipment is made, paperwork must be done and an invoice made out. Billing
and collecting costs can be reduced by consolidating shipments and reducing the pickup
frequency.
Total Transportation Costs
The total cost of transportation consists of line haul, pickup and delivery, terminal-handling, and billing and collecting costs. To reduce shipping costs, the shipper needs to do
the following:
Decrease line haul costs per unit by increasing the weight shipped.
Decrease pickup and delivery cost by reducing the number of pickups. This can be
done by consolidating and increasing the weight per pickup.
■■ Decrease terminal-handling costs by decreasing the number of parcels by consolidating shipments.
■■ Decrease billing and collecting costs by consolidating shipments.
■■
■■
For any given shipment, the line haul costs vary with the distance shipped. However,
the pickup and delivery, terminal-handling, and billing costs are fixed. The total cost
for any given shipment thus has a fixed cost and a variable cost associated with it. This
relationship is shown in Figure 13.4. The carrier will consider this relationship and either
charge a fixed cost plus so much per mile or offer a tapered rate. In the latter, the cost per
mile for short distances far exceeds that for longer distances.
The rate charged by a carrier will also vary with the commodity shipped and will
depend upon the following:
Value. The more valuable the item, the greater the carrier’s liability for damage will be.
Density. The more dense the item, the greater the weight that can be carried in a
given vehicle.
■■ Perishability. Perishable goods often require special equipment and methods of
handling.
■■ Packaging. The method of packaging influences the risk of damage and breakage.
■■ Hazards. Loads containing hazardous materials may require special considerations
and/or handling.
■■
■■
In addition, carriers have two rate structures, one based on full loads called truckload
(TL) or carload (CL) and one based on less than truckload (LTL) and less than carload
Total Cost
COST
324
Variable Cost
Fixed Cost
DISTANCE
Figure 13.4 Distance versus cost of transportation.
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325
(LCL). For any given commodity, the LTL rates can be up to 100% higher than the TL
rates. The basic reason for this differential lies in the extra pickup and delivery, terminalhandling, and billing and collection costs. Truckers, airlines, and water carriers accept less
than full loads, but rail companies usually do not accept LCL shipments.
Transportation Terms
The transporters of goods have used terminology for many years to specify who pays the
cost of transportation and who owns the goods in-transit. The four basic terms in common
use are based on the term FOB, freight on board, which specifies who is responsible for
the goods and pays the insurance to protect against risk for the goods.
The four FOB terms used allow astute buyers and sellers to take advantage of their
own area of expertise to keep costs as low as possible. A small company with a low shipping volume often pays higher transportation fees than a large supplier. In the case of a
small buyer, it is often better to get the shipper to pay for transportation and bill the buyer
at a lower total cost. A company with their own fleet of trucks would want to take responsibility for the freight charges.
FOB origin, freight collect. The seller makes the goods available at their dock and
all risk and costs of transportation are assumed at this point by the buyer.
■■ FOB origin, freight pre-paid.
The seller delivers the goods to the buyer but the
buyer has the risk of the goods while in-transit and pays the insurance.
■■ FOB destination, freight pre-paid.
The seller is responsible for all costs of risk and
transportation to the buyer’s location.
■■ FOB destination, freight collect.
The seller is responsible for the risk to the buyer’s
location but the buyer is responsible for the freight.
■■
Incoterms
The term FOB indicates the point of transfer of ownership for North American shipments
only, which does not necessarily conform to the international standards of Incoterms.
International Commercial Terminology (Incoterms) was developed by the International
Chamber of Commerce in 1936 to establish standards governing the responsibilities
between buyers and sellers when transporting goods within and across international boundaries. The terms have been subsequently updated, and the definitions in this text are based
on Incoterms 2010. In-transit goods represent a major investment to buyer and seller and
run many risks of damage or loss. It is essential that a clear understanding exists as to who
actually owns the goods at a given time or location in the supply chain and who will pay the
expenses incurred to transport the goods. Another risk when transporting hazardous goods
is the potential for accidents or spills and the potential environmental cleanup costs.
There are three responsibilities to be addressed:
Cost of transportation of the goods.
Ownership or insurance against risk of the goods.
■■ Preparation of customs documentation for the movement of the goods.
■■
■■
Incoterms are divided into two groups:
■■
■■
Rules for any mode or modes of transport.
Rules for sea and inland waterway transport.
Terms for any mode(s) of transport
EXW (named place): Ex Works. The seller makes the goods available at their
premises. The buyer is responsible for all charges to transport and insure the goods,
including customs documentation. The buyer bears all risks for transporting the goods.
■■ FCA (named carrier): Free carrier.
The seller is responsible for preparing the
export documentation and delivering the goods to the transporter named after the
Incoterm. The risk then passes to the buyer, who takes responsibility for all subsequent
transportation costs.
■■
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CPT (named destination port): Carriage paid to. The seller is responsible for
export documentation and pays the freight to the named destination. However, the risk
passes to the buyer when the goods have been delivered to the first carrier.
■■ CIP (named destination): Carriage and insurance paid to. The seller pays the costs
of transportation to the destination and prepares the export documentation. Risk transfers to the buyer when the goods have been delivered to the first carrier.
■■ DAT (named place): Delivered at terminal.
The seller prepares the export documentation and bears transportation costs to deliver the goods to the named terminal or
port. Both parties must agree at which point within the terminal the risk is transferred
from seller to buyer.
■■ DAP (named place): Delivered at place.
The seller delivers the goods to the buyer,
who assumes responsibility for the unloading at the named place. Both parties must
agree at which point at the destination place the risk is transferred from seller to buyer.
■■ DDP (named destination): Delivered duty paid.
The seller pays all the costs of
shipping, export documentation, and any duties to the named destination, which is typically the buyer’s location.
■■
Terms for Sea and Inland Waterway Transport
FAS (named ship): Free alongside ship. The seller is responsible for export documentation and the transportation of the goods to the point of shiploading at the port of
export. Risk is passed to the buyer once the goods are delivered to the port of shipment.
The buyer pays loading costs, freight, insurance, unloading costs, and transportation
from the import port.
■■ FOB (named ship): Free on board.
The seller is responsible for export documentation and delivery of the goods onto the named vessel or “past the ship’s rail.” FOB destination designates that the seller is responsible until the buyer takes possession. Note
that this term is similar but not the same as the term FOB introduced earlier for North
American freight.
■■ CFR (named destination port): Cost and freight. The seller is responsible for
export documentation and pays for the freight to the port of destination. However,
when the goods “pass the ship’s rail” in the port of shipment, the buyer assumes the
risk for the goods. This Incoterm is intended for use when shipping product that is not
containerized.
■■ CIF (named destination port): Cost, insurance, and freight. The selling price
includes the cost of all transportation costs and insurance to the named destination port.
The seller is responsible for acquiring the insurance policy, but the responsibility of the
goods ends when the goods are aboard the vessel.
■■
Warehousing
Chapter 12 discussed the management of warehouses. This section is concerned with the
role of warehouses in a physical distribution system.
Warehouses include factory warehouses, regional warehouses, and local warehouses. They may be owned and operated by the supplier or intermediaries such as
wholesalers, or they may be public warehouses. The latter offer a general service to
their public, which includes providing storage space and warehouse services. Some
warehouses specialize in the kinds of services they offer and the goods they store. A
freezer storage is an example. The service functions that warehouses perform can be
classified into two kinds:
1. The general warehouse is where goods are stored for long periods and where the
prime purpose is to protect goods until they are needed. There is minimal handling,
movement, and relationship to transportation. Furniture storage or a depository for
documents are examples of this type of storage. It is also the type used for inventories
accumulated in anticipation of seasonal sales.
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327
2. The distribution warehouse has a dynamic purpose of movement and mixing.
Goods are received in large volume uniform lots, stored briefly, and then broken
down into small individual orders of different items required by the customer in the
marketplace. The emphasis is on movement and handling rather than on storage. This
type of warehouse is widely used in distribution channels. The size of the warehouse
is not so much its physical size as it is the throughput, or volume of traffic handled.
As discussed in the previous chapter, warehouses, or distribution centers, are places
where raw materials, semi-finished, or finished goods are stored. They represent an interruption in the flow of material and thus add cost to the system. Items should be warehoused only if there is an offsetting benefit gained from storing them.
Role of Warehouses
Warehouses serve three important roles: transportation consolidation, product mixing, and
service.
Transportation consolidation As shown in the preceding section, transportation
costs can be reduced by using warehouses. This is accomplished by consolidating small
(LTL) shipments into large (TL) shipments.
Consolidation can occur in both the supply and distribution systems. In physical
supply, LTL shipments from several suppliers can be consolidated at a warehouse before
being shipped as a TL to the factory. In physical distribution, TL shipments can be made
to a distant warehouse and LTL shipments made to local users. Figure 13.5 shows the two
situations graphically. Transportation consolidation in physical distribution is sometimes
called break-bulk, which means the bulk (TL) shipments from factories to distribution
centers are divided into small shipments going to local markets. Cross-docking may also
be used, meaning the items are moved from the incoming docking area directly to the outgoing carrier without being stored at the warehouse.
Product mixing Although transportation consolidation is concerned with reduction
of transportation costs, product mixing deals with the grouping of different items into an
order and the economies that warehouses can provide in doing this. When customers place
orders, they often want a mix of products that are produced in different locations. The
PHYSICAL SUPPLY SYSTEM
SUPPLIER A
LTL
Shipments
SUPPLIER B
TL Shipments
(A, B, C)
WAREHOUSE
FACTORY
SUPPLIER C
PHYSICAL DISTRIBUTION SYSTEM
FACTORY A
TL
Shipments
FACTORY B
FACTORY C
Figure 13.5 Transportation consolidation.
LTL
Shipments
WAREHOUSE
Break-bulk
M
A
R
K
E
T
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same can be true for shipments being made from a distribution center to a retailer. Rather
than ordering many items from multiple sources, fulfillment can be done through a distribution center that mixes the various products supplied from multiple sources.
Without a distribution center, customers and retailers would have to order from each
source and pay for LTL transport from each source. Using a distribution center, orders can
be placed and delivered from a central location. Figure 13.6 illustrates the concept.
Service Distribution centers improve customer service by positioning goods close to
markets so the markets can be served more quickly.
Warehousing and Transportation Costs
Any distribution channel should try to provide the highest service level (the number of
orders delivered in a specified time) at the lowest possible cost. The particular shipping
pattern will depend largely upon the following:
Number of customers.
Geographic distribution of the customers.
■■ Customer order size.
■■ Number and location of plants and distribution centers.
■■
■■
Suppliers have little or no control over the first three but do have some control over
the last. They can establish local distribution centers in their markets. With respect to
transportation, it then becomes a question of the cost of serving customers directly from
the central distribution center or from a regional distribution center. If truckload shipments
are made, the cost is less from the central distribution center, but if LTL shipments are
made, it may be cheaper to serve the customer from the local distribution center.
example Problem
Suppose a company with a plant located in Toronto, Canada, is serving a market in the
northeastern United States with many customers located in Boston. If the company ships
direct to customers from the Toronto plant, most shipments will be less than truckload.
However, if it locates a distribution center in Boston, it can ship truckloads to Boston and
distribute by local cartage (LTL) to customers in that area. Whether this is economical or
not depends on the total cost of shipping direct compared to shipping via the distribution
center. Assume the following figures represent the average shipments to the Boston area:
Plant to customer LTL: $100/cwt.
Plant to distribution center TL: $50/cwt.
Inventory carrying cost (distribution center): $10/cwt.
Distribution center to customer LTL: $20/cwt.
Is it more economical to establish the distribution center in Boston? If the annual
shipped volume is 10,000 cwt., what will be the annual saving?
MANUFACTURER
A
ts
uc
od
Pr , B, C
A
od
Pr
tA
uc
Product B
od
uc
t
C
MANUFACTURER
B
MANUFACTURER
C
Pr
328
Figure 13.6 Product mixing.
CUSTOMER
Y
DISTRIBUTION
CENTER
Pr
o
A, duc
B, ts
C
CUSTOMER
Z
Physical Distribution
329
Answer
Costs if a distribution center is used:
TL Toronto to Boston = $50 per cwt.
Distribution center costs = $10 per cwt.
LTL in Boston area = $20 per cwt.
Total cost = $80 per cwt.
Saving per cwt. = $100 - $80 = $20
Annual saving = $20 * $10,000 = $200,000
Market Boundaries
Continuing with the previous example problem, the company can now supply customers
in other locations directly from the factory in Toronto or through the distribution center in
Boston. The question is to decide which locations should be supplied from each source.
The answer, of course, is the source that can service the location at least cost.
In order to determine least cost, the logistics costs from different locations need to
be determined. Landed cost, sometimes referred to as total landed cost, laid down cost
or landed duty price, is the delivered cost of a product to a particular geographic point.
The delivered cost includes all logistics costs of moving the goods from A to B, including
transportation, warehousing, handling, and any taxes and fees associated with logistics.
In the previous example problem, the landed cost of delivering from Toronto would be
the transportation cost per mile times the miles to a particular destination. The landed
cost from Boston would include all costs of getting the goods to Boston, inventory costs
in the Boston distribution center, and the transportation costs in getting to a particular
destination.
Landed Cost = P + TX + F
where
P = product costs
T = transportation costs per mile
X = distance
F = fees
The product cost includes all costs in getting the product to the supply location and
storing it there. In the previous example, the product cost at Boston includes the TL cost of
delivery to Boston and the inventory cost at Boston.
example Problem
Syracuse is 300 miles from Toronto. The product cost for an item is $10 per cwt., and
the transportation cost per mile per cwt. is $0.20. What is the landed cost per cwt.?
Answer
Landed cost = Product cost + (transportation cost per mile)(distance)
= $10 + 1$0.20 * 3002 = $70 per cwt.
Market boundary The market boundary is the line between two or more supply
sources where the landed cost is the same. Consider Figure 13.7. There are two sources of
X
Miles
A
Figure 13.7 Market boundary.
100
Miles
(100 – X )
Miles
Y
B
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Chapter thirteen
supply: A and B. The market boundary occurs at Y, where the landed cost from A is the
same as from B.
In the example shown in Figure 13.7, the distance between A and B is 100 miles. If
the distance from A to Y is X miles, then the distance from B to Y is 1100 - X2 miles.
Assume supply A is the factory and supply B is a distribution center. Assume the product
cost at A is $100 and product cost from B is $100 plus TL transportation from A to B
and inventory costs at B. For this example, assume the TL transportation and inventorycarrying costs are $10 per unit so the product cost from B is $110. Transportation costs
from either A or B are $0.40 per unit per mile.
Point Y occurs where
Cost A = Cost B
100 + 0.40X = 110 + 0.401100 - X2
X = 62.5
Thus, a point Y, 62.5 miles from A, marks the market boundary between A and B.
example Problem
The distance between Toronto and Boston is about 500 miles. Given the cost structure in the previous example problems and an LTL transportation cost of $0.20 per
cwt. per mile, calculate the location of the market boundary between Toronto and
Boston. Assume the product cost at Toronto is $10 per cwt.
Answer
The product cost at Boston is the sum of the product cost at Toronto, plus the cost of
TL shipment from Toronto to Boston, plus the handling costs at Boston.
Product cost at Boston = product cost at A + TL transportation + handling costs
= $10 + $50 + $10
= $70
The market boundary occurs where
Cost from Toronto = Cost from Boston
$10 + $0.20X = $70 + $0.201500 - X2
0.4X = 160
X = 400
The market boundary is 400 miles from Toronto or 100 miles from Boston.
Effect on Transportation Costs of Adding More Warehouses
The previous example showed that establishing a distribution center in Boston reduces
total transportation costs. Similarly, if a second distribution center is established, perhaps
in Cleveland, Ohio, one would expect total transportation costs to be reduced further.
Generally, as more distribution centers are added to the system, the following can be
expected:
The cost of truckload (and carload) shipments to the distribution centers will increase.
The cost of LTL shipments to customers will decrease.
■■ The total cost of transportation will decrease.
■■
■■
As expected, the major savings is from the addition of the first few distribution centers. Eventually, as more distribution centers are added, the savings decrease. The first
distribution center added to the channel is located to serve the largest market; the second
distribution center, the second largest market, and so on. The number of customers served
by additional distribution centers decreases, and the volume that can be shipped TL to
the additional distribution centers is less than to the first distribution centers. Figure 13.8
shows the relationship that exists between transportation costs and the number of distribution centers in a system.
331
TRANSPORTATION COST IN DOLLARS
Physical Distribution
NUMBER OF WAREHOUSES
Figure 13.8 Transportation cost versus number of warehouses.
Packaging
The basic role of packaging in any industrial organization is to carry the goods safely
through a distribution channel to the customer. The package must do the following:
Identify the product.
Contain and protect the product.
■■ Contribute to physical distribution efficiency.
■■
■■
For consumer products, the package may also be an important part of the marketing
program.
Physical distribution must not only move and store products but also identify them. The
package serves as a means of identifying the product in a way not possible from its outward
appearance. When shoes are offered in 10 sizes, the package becomes an important identifier.
Packaging must contain and protect the product, often against a wide range of hazards such as shock, compression, vibration, moisture, heat, solar radiation, oxidation, and
infestation by animals, insects, birds, mold, or bacteria. Packages are subject to distribution hazards in loading and offloading, in movement, in transportation, and in warehousing and storage. The package must be robust enough to protect and contain the product
through all phases of distribution.
Packaging is a cost that must be offset by the increased physical distribution efficiency that the package can provide.
There are usually at least three levels of packaging required in a distribution system. First
is a primary package that holds the product, for example, a box of cereal. Next, for small packages, a shipping container such as a corrugated box is needed. Finally, there is a third level of
packaging, where several primary or secondary packages are assembled into a unit load.
Unitization
Unitization is the consolidation of several units into large units, called unit loads, so
there is less handling. A unit load is a load made up of a number of items, or bulky material, arranged or constrained so the mass can be picked up or moved as a single unit too
large for manual handling. Material handling costs decrease as the size of the unit load
increases. It is more economical to move the product by cartons rather than individually
and still more economical to move several cartons in one unit load.
There are a number of unit-load devices such as sheets, racks, and containers. One of
the most common is the pallet.
As noted in Chapter 12, the pallet is a platform usually measuring 48″ * 40″ * 4″
and designed so that it can be lifted and moved by an industrial truck called a forklift.
Packages are arranged on it so that several packages may be moved at one time. Loaded
with packages, it forms a cube that is a unit load.
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Chapter thirteen
Unitization can be successive. Shippers place their products into primary packages,
the packages into shipping cartons, the cartons onto pallets, and the pallets into warehouses, trucks, or other vehicles.
To use the capacity of pallets, trucks or other vehicles, and warehouses, there should
be some relationship between the dimensions of the product, the primary package, the
shipping cartons, the pallet, the truck, and the warehouse space. The packages should be
designed so space on the pallet is fully utilized and so the cartons interlock to form a stable
load. Figure 13.9 shows two unit loads, each using the total space of the pallet. However,
load B does not interlock and is not stable.
Pallets fit into trucks and railway cars. The standard dimensions are selected so pallets would fit into nominal 50' railway cars and 40' or 53' truck trailers with a minimum of
lost space. Figure 13.10 shows the layout in railcars and trailers.
Load A
Load B
Figure 13.9 Stable and unstable pallet loads.
Nominal 50' Railcar
48"
48"
Nominal 40' Trailer
40"
40"
56 Pallets
Full CL
2 Tiers
40 Pallets
Full TL
2 Tiers
92"
110"
Figure 13.10 Railcar and trailer pallet position plan.
Physical Distribution
333
Thus, to get the highest cube utilization, consideration must be given to the dimensions of the product, the carton, the pallet, the vehicle, and the warehouse.
material hanDling
Material handling is the short-distance movement that takes place in or around a building
such as a plant or distribution center. For a distribution center, this means the unloading
and loading of transport vehicles and the dispatch and recall of goods to and from storage.
In addition, the racking systems used in distribution centers are usually considered to be
part of material handling.
Some objectives of material handling are as follows:
1. To increase cube utilization by using the height of the building and by reducing the
need for aisle space as much as possible.
2. To improve operating efficiency by reducing handling. Increasing the load per move
will result in fewer moves.
3. To improve the service level by increasing the speed of response to customer needs.
There are many types of material handling equipment. For convenience, they can be
grouped into three categories: conveyors, industrial trucks, and cranes and hoists.
Conveyors are devices that move material (or people) horizontally or vertically
between two fixed points. They are expensive, create a fixed route, and occupy space
continuously. As a result, they are used only where there is sufficient throughput between
fixed points to justify their cost.
Industrial trucks are vehicles powered by hand, electricity, or propane. Diesel and
gasoline are not used indoors because they are noxious and lethal. Industrial trucks are
more flexible than conveyors in that they can move anywhere there is a suitable surface
and no obstructions. They do not occupy space continuously. For these reasons, they are
the most often used form of material handling in distribution centers and in manufacturing.
Cranes and hoists can move materials vertically and horizontally to any point within
their area of operation. They use overhead space and are used to move heavy or large
items. Within their area of operation, they are very flexible.
multi-Warehouse systems
This section will look at the result of adding more distribution centers to the system. As
might be expected, there is an effect on the cost of warehousing, material handling, inventories, packaging, and transportation. The objective will be to look at how all of these
costs and the total cost behave, as well as what happens to the service level as more distribution centers are added to the system.
Transportation Costs
In the section on transportation, it was discussed that if shipments to customers are in lessthan-full vehicle lots, the total transportation cost is reduced by establishing a distribution
center in a market area. This is because more weight can be shipped for greater distances
by truck or carload and the LTL shipments can be made over relatively short distances.
Generally, then, as more distribution centers are added to a system, the following can be
expected:
The cost of TL shipments increases.
■■ The cost of LTL shipments decreases.
■■ The total cost of transportation decreases.
■■
The major savings are made with the addition of the first distribution centers.
Eventually, as more distribution centers are added, the marginal savings decrease.
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Chapter thirteen
Inventory Carrying Cost
The average inventory carried depends on the order quantity and the safety stock. The
average order quantity inventory in the system should remain the same since it depends on
demand, the cost of ordering, and the cost of carrying inventory.
The total safety stock will be affected by the number of warehouses in the system.
Safety stock is carried to protect against fluctuations in demand during the lead time and
depends, in part, on the number of units sold. In Chapter 11, it was shown that the standard deviation varies as the square root of the ratio of the forecast and lead time intervals.
Similarly, for the same SKU, the standard deviation varies approximately as the square
root of the ratio of the different annual demands. Suppose that the average demand is
1000 units and, for a service level of 90%, the safety stock is 100 units. If the 1000 units
is divided between two distribution centers each having a demand of 500 units, the safety
stock in each is:
SS = 100
500
= 71 units 1in each warehouse2
A 1000
With two distribution centers and the same total sales, the total safety stock increases
to 142 from 100. Thus, with a constant sales volume, as the number of distribution centers
increases, the demand on each decreases but there is an increase in the total safety stock in
all distribution centers.
Warehousing Costs
The fixed costs associated with distribution centers are space and material handling. The
space needed depends on the amount of inventory carried. As noted previously, as more
distribution centers are added to the channel, more inventory has to be carried, which
requires more space.
In addition, there will be some duplication of nonstorage space such as restrooms and
offices. So, as the number of distribution centers increases, there will be a gradual increase
in distribution center space costs.
Operating costs also increase as the number of distribution centers increases.
Operating costs depend largely on the number of units handled. If there is no increase in
sales, the total number of units handled remains the same, as does the cost of handling.
However, the nondirect supervision and clerical costs increase.
Material Handling Costs
Material handling costs depend upon the number of units handled. If the sales volume
remains constant, the number of units handled should also remain constant. There will be
little change in material handling costs as long as the firm can ship unit loads to the distribution center. However, if the number of distribution centers increases to the point that
some nonunitized loads are shipped, material handling costs increase.
Packaging Costs
Per-unit packaging costs will remain the same, but since there will be more inventory in
the multi-warehouse system, total packaging costs will rise with inventory.
Total Cost
With the assumption that total sales remain the same, Figure 13.11 shows graphically
how the costs of transportation, warehousing, material handling, inventory, and packaging
behave as distribution centers are added to the system. Up to a point, total costs decrease
and then start to increase. Similar to the determination of an economic order quantity
(EOQ), as discussed in Chapter 10, it is the objective of logistics to determine this leastcost point.
Physical Distribution
Total Cost
Inventory
Packaging
Warehouse
Materials
Handling
TOTAL COST IN DOLLARS
Transp.
335
NUMBER OF WAREHOUSES
Figure 13.11 Total system cost.
System Service Capability
The service capability of the system must also be evaluated. One way of assessing this
is by estimating the percentage of the market served within a given period. Figure 13.12
represents such an estimate.
As expected, the service level increases as the number of distribution centers
increases. It increases rapidly from one to two distribution centers and much less rapidly
as the number is further increased. The first distribution center is built to serve the best
market, the next to serve the second-best market, and so on.
Assume that a study has been made of a system of 1 to 10 distribution centers and the
costs are as shown in Figure 13.13.
A three-distribution-center system would provide the least total cost. Figure 13.13
shows that by moving from 3 to 10 distribution centers, the 1-day service level increases
by 8%. Management must decide which distribution system to select. The decision must
be based on adequate analysis of the choices available and a comparison of the increase in
costs and service level.
Number of
Warehouses
Percentage of Market
Reached in 1 Day
1
2
3
10
30
70
87
95
Figure 13.12 Estimate of market reached versus number of warehouses.
Cost ($1000)
Transportation
Warehousing
Materials Handling
Inventory
Packaging
Total Cost
1
$8000
500
1000
400
100
Number of Locations
2
3
$6000
$5000
600
700
1000
1100
425
460
100
100
$10,000
Figure 13.13 Cost versus number of warehouses.
$8125
$7360
10
$4500
900
1400
700
100
$7600
336
Chapter thirteen
summary
Physical distribution is the movement of goods into and out of a company and is changing in both the global economy and in the individual delivery of goods to internet
customers. The majority of goods are still shipped in large economical units through
distribution channels. Third and fourth party logistics providers are applying their area
of specialization to help companies drive down costs while improving customer service.
Sound packaging design is critical to a company not only because it affects the efficient
handling of product all the way through the supply chain but also because it can have a
high environmental cost. Reverse logistics along with a change in the distribution channel to internet customers is another reason for the growth of 3PLs. Warehouses strategically placed through the supply chain can help provide improved customer service and
reduce costs.
The global sourcing and distribution of goods across international boundaries has
increased the need for accurate, comprehensive documentation to determine things such
as who pays the freight, who prepares the documentation, who owns the goods, and who
may be responsible for any environmental problems. Incoterms, standard terms accepted
around the world, address most of these concerns and are a requirement of any freight
movement. Scheduling of shipments is a critical part of physical distribution in order
to optimize transportation methods while taking speed, cost, and customers’ needs into
consideration.
key terms
Backhauling 320
Billing and collecting costs 324
Birdyback 320
Break-bulk 327
Common carriers 320
Contract carriers 321
Conveyors 333
Cost trade-off 315
Cranes and hoists 333
Cross-docking 327
Density 324
Distribution channel 312
Distribution inventory 314
Distribution warehouse 327
Fishyback 320
Fixed costs 318
Fourth party logistics provider (4PL) 317
Freight on board (FOB) 325
General warehouse 326
Green reverse logistics 313
Hazards 324
Incoterms 325
Intermodal 320
Industrial trucks 333
Landed cost 329
Less than truckload (LTL) 324
Line haul cost 322
Line haul cost per hundred weight 322
Market boundary 329
Material handling 315
Order processing and communication 315
Packaging 324
Pallet 331
Perishability 324
Physical distribution 311
Physical supply 311
Pickup and delivery costs 323
Piggyback 320
Private carriers 321
Protective packaging 315
Reverse logistics 313
Terminal-handling costs 323
Terminals 318
Third party logistics provider (3PL) 316
Total cost 315
Total line haul cost 322
Transaction channel 312
Transportation 314
Transportation management
system (TMS) 320
Truckload (TL) 324
Unit loads 331
Physical Distribution
Unitization 331
Value 324
Variable costs 318
337
Vehicles 318
Warehouses (distribution centers) 314
Ways 318
Questions
1. Name and describe the two functions in the flow of materials from supplier to consumer. What
are the differences between physical supply and physical distribution?
2. What is the primary function of the transaction channel and the distribution channel?
3. The particular way that goods move depends in part on four factors. What are they?
4. Describe the reverse logistics system of a beverage company, including the movement of goods
and the flow of payment. What steps does the company take to reduce their costs?
5. Why are the total costs associated with reverse logistics increasing? Will this trend continue
into the future?
6. What are the objectives of a physical distribution system?
7. Name and describe each of the six activities in a physical distribution system.
8. What are the cost trade-off and total cost concepts? Why are they important?
9. Describe the relationship between marketing and physical distribution. How does physical distribution contribute to creating demand?
10. Why is the demand placed on a central distribution center or a factory by distribution centers
considered dependent?
11. What are the five basic modes of transportation?
12. What are the three physical elements in a transportation system? For each of the five modes,
describe who provides them and how they are funded.
13. Describe why train service is cheaper than road transport for large quantities of bulky commodities moving over long distances.
14. Why can trucks provide a fast, flexible service for the distribution of small volumes of goods
to a dispersed market?
15. What are the major characteristics of water and air transport?
16. What are the major legal types of carriage? What are the three areas of economic regulation?
To which legal type of carriage do they apply?
17. Compare common and contract carriage. How do they differ from private carriage? Which will
give the highest level of service?
18. On what do total line haul costs and line haul costs per hundredweight depend? What two ways
can shippers reduce line haul costs?
19. Describe how a shipper can reduce the following:
a. Pickup and delivery costs.
b. Terminal-handling costs.
c. Billing and collecting costs.
20. The rates charged by a shipper vary with the commodity shipped. Name and describe five factors that affect the rates.
21. Why are LTL rates more expensive than TL rates?
22. Name and describe the two basic types of warehouses.
23. Name and describe the three important roles warehouses serve.
24. Name four factors that affect shipping patterns. Which can a supplier control?
25. What is the landed cost? What is a market boundary? Why are landed costs important in determining market boundaries?
26. As more distribution centers are added to a system, what happens to the cost of truckload, less
than truckload, and total transportation costs?
27. What are the three roles of packaging in a distribution system? Describe why each is important.
28. What is unitization? Why is it important in physical distribution? Why is it successive?
29. What are three prime objectives of material handling? Describe the characteristics of conveyors, industrial trucks, cranes, and hoists.
338
Chapter thirteen
30. As more warehouses are added to the system, what would we expect to happen to the
following?
a. Transportation costs.
b. Inventory costs.
c. Material handling costs.
d. Packaging costs.
e. Total costs.
f. System service capability.
31. How does the use of the transportation term FOB for domestic shipments in North America
differ from the use of the term in international shipments?
32. Which North American shipping term has the minimum risk for:
a. the buyer?
b. the seller?
33. A buyer in England would like to import a Harley–Davidson motorcycle directly from a dealer
in the United States. Which Incoterm would minimize the buyer’s efforts? Would arranging the
shipment themselves save money for the buyer?
34. What is meant by the term green reverse logistics?
35. Describe how a 3PL would benefit a company that sells their product to individuals through the
internet.
36. Describe how reverse logistics would apply to your school bookstore.
Problems
13.1. A company normally ships to a customer by rail at a cost of $500 per load. The transit
time is 14 days. The goods can be shipped by truck for $700 per load and a transit time
of 4 days. If transit inventory cost is $35 per day, what does it cost to ship each way?
Answer.
Rail, $990; truck, $840
13.2. A company manufactures component parts for machine tools in North America
and ships them to Southeast Asia for assembly and sale in the local market. The
components are shipped by sea, transit time averages 6 weeks, and the shipping
costs $2700 per shipment. The company is considering moving the parts by air at
an estimated cost of $7500; the shipment taking 2 days to get there. If inventory
carrying cost for the shipment in transit costs $100 per day, should they ship by air?
Why should the fact that forecasts are more accurate for nearer periods of time be
considered? What activities are affected by the shorter lead time?
13.3. For a given commodity, the line haul cost is $13 per mile. For a trip of 200 miles
and a shipment of 300 lbs., what is the cost per pound? If the shipment is increased
to 500 lbs., what is the saving in cost per pound?
Answer.
$3.47 per lb.
13.4. A company ships a particular product to a market located 1500 miles from the plant
at a cost of $4.50 per mile. Normally it ships 500 units at a time. What is the line
haul cost per unit?
13.5. In problem 13.4, if the company can ship the units unassembled, it can ship 750
units in a truck. What is the line haul cost per unit now?
13.6. A company processes feathers and ships them loose in a covered truck. The line
haul cost for an average shipment is $600, and the truck carries 2000 pounds of
feathers. A bright new graduate has just been hired and has suggested that they
should bale the feathers into 500-pound bales. This would make them easier to
handle and also allow them to be compressed into about one-tenth of the space they
now occupy. How many pounds of feathers can the truck now carry? What is the
present line haul cost per pound? What will it be if the proposal is adopted?
Physical Distribution
339
13.7. A company in Calgary serves a market in the northwestern United States. Now it
ships LTL at an average cost of $30 per unit. If the company establishes a distribution
center in the market, it estimates the TL cost will be $15 per unit, inventory carrying
costs will be $7 per unit, and the local LTL cost will average $6 per unit. If the company forecasts annual demand at 200,000 units, how much will they save annually?
Answer.
Annual saving = $400,000
13.8. A company ships LTL to customers in a market in the Midwest at an average cost
of $40 per cwt. It proposes establishing a distribution center in this market. TL
shipment costs to the DC would be $20 per cwt., the estimated inventory carrying
costs are $5 per cwt., and the local cartage (LTL) cost is estimated at $10 per cwt.
If the annual shipped volume is 100,000 cwt., what will the annual savings be by
establishing the distribution center?
13.9. A company has a central supply facility and a distribution center located 400
miles away. The central supply product cost is $75, TL transportation rates from
central supply to the DC are $60 per unit, and handling costs at the DC are $4 per
unit. Calculate the market boundary location and the landed cost at the market
boundary. LTL rates are $2 per unit per mile.
Answer.
Market boundary is 216 miles from central supply. Landed cost = $507
13.10. Suppose the company in problem 13.8 had another market area located between
the parent plant and the proposed distribution center. The LTL costs from the
plant to that market are $35 per cwt. The company estimates that LTL shipments
from the distribution center will cost $6 per cwt. Should it supply this market
from the distribution center or central supply?
13.11. A company can ship LTL direct to customers in city A or use a public warehouse
located in city B. It has determined the following data.
Cost per pound for shipping LTL to city A is $0.70 + $0.30 per mile.
Cost per pound for shipping TL to warehouse is $0.40 + $0.15 per mile.
Warehouse handling costs are $0.30 per pound.
Distances: Plant to city A = 115 miles
Plant to city B = 135 miles
From city B to city A = 30 miles
a. What is the total cost per pound to ship from the plant direct to customers in
city A?
b. What is the total cost per pound to ship via the warehouse in city B?
c. In this problem, the cost per pound has a fixed and a variable component. Why?
case stuDy 13.1
Metal Specialties, inc.
Metal Specialties is a wholesaler of specialty metals such as stainless steel and tool steel.
The company purchases its stainless steel from a mill located some 200 miles away. At
present the company operates its own truck. However, the truck is in need of repairs estimated at $20,000. Annual operating costs are $30,000 and the line haul costs are $2.20
per mile. Janet Jones (JJ), the traffic manager, wants to reduce the cost of bringing in
the stainless steel, and because of the impending repair expense, she feels now is a good
time to look at alternatives. She has solicited a number of proposals and has narrowed her
choices down to a motor carrier and a rail carrier.
340
Chapter thirteen
Heavy Metal Transport (HMT), a contract motor carrier, has an excellent reputation
for service and reliability. It has submitted an incremental rate, $4.00/cwt. for shipments
weighing less than 150 cwt., $3.80 for shipments between 150 and 200 cwt., $3.60 for
shipments between 200 and 250 cwt., and $3.40 for shipments over 250 cwt. up to a maximum of 400 cwt.
Midland Continental Railway has submitted a piggyback rate of $3.25 per cwt. with
a minimum load of 200 cwt. The piggyback rate includes pickup by truck at the steel mill,
line haul by trailer on flat car, and delivery by truck to Metal Specialities’ warehouse.
They are considered to be a reliable carrier as well.
The finance department estimates that Metal Specialities’ annual inventory carrying
cost is 20%, the cost of inventory in transit is 10%, and the cost of capital is 8%. The cost
of placing an order for stainless steel is estimated to be $40 per order. Stainless steel presently costs $300 per cwt.
assignment
1. JJ has to make a decision soon. Given the information provided, what would you
advise her to do?
Chapter
FOURTEEN
PROdUcTs aNd PROcEssEs
INTROdUcTION
The effect and the efficiency of operations management, lean manufacturing, and total quality management all depend on the way products are designed and the processes selected.
The way products are designed determines the processes that are available to make them.
The product design and the process determine the quality and cost of the product. Quality
and cost determine the profitability of the company. This chapter studies the relationship
between product design and process design and the costs associated with different types of
processes. Finally, the chapter looks at the improvement of existing processes.
NEEd FOR NEw PROdUcTs
Products, like people, have a limited life span. A product passes through several stages,
known as the product life cycle, beginning with its introduction and ending with its disappearance from the marketplace. Figure 14.1 gives a simplified view of the profit and volume relationships in each phase of the cycle. No time scale is implied. The life cycle may
take months or years to complete depending on the products and the market.
Introduction phase This phase is the most expensive and risky stage. To get customer acceptance of the product, the firm will usually spend heavily on advertising and
sales promotion, hoping these costs will be recovered in future sales. If the introduction
fails, the firm loses money, a fact that underlines the importance of thoroughly researching
a new product before introducing it.
Growth phase In this phase, sales of a successful product increase at a rapid rate. The
design of the product has stabilized and production increases, resulting in a decrease of the
unit cost of the product. The increased sales volume and the lower unit cost cause profits
to increase rapidly. However, the success of the product usually attracts the attention of
INTRODUCTION
GROWTH
MATURITY
DECLINE
SALES
PROFIT
0
TIME
Figure 14.1 Life cycle of a product.
341
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competitors. Their entrance into the market forces prices down, possibly reducing the
firm’s sales. At this point profits are squeezed.
Maturity or saturation phase Nearly everyone interested in the product has sampled or owns the product and sales begin to level off. The market is saturated, price competition is often severe and profits start to decline.
Decline phase Sales drop as customers begin to lose interest in the product or to buy
improved versions from the company or its competitors. As profits decline still further,
companies will look for ways to maintain profitability. Generally, there are three ways in
which this can be done:
Introduce new products.
Improve existing products.
■■ Improve the methods of production.
■■
■■
Depending on the firm’s resources, it may do these things through its own research
and development, by copying competitors’ products, or by relying on customers or suppliers to do the research and development work. This discussion focuses on the firm that
does its own research, development, and engineering.
PROdUcT dEvElOPmENT PRINcIPlEs
A few organizations supply a single product, but most supply a range of similar or related
products. There are two conflicting factors to be considered in establishing the range of
products to supply.
■■
■■
If the product line is too narrow, customers may be lost.
If the product line is too wide, customers may be satisfied, but operating costs will
increase because of the lack of specialization.
Sales organizations are responsible for increasing sales and revenue. They want to
offer product variety to consumers. Often this means the organization must offer a variety
of products, many of which sell in small volumes.
Operations, on the other hand, would like to produce as few products as possible and
make them in long runs. In this way it can reduce the number of setups (and cost) and
probably reduce run costs by using specialized equipment or labor. It would fulfill its
mandate to produce at the lowest cost.
Somehow the needs of the market and the economics of production must be balanced.
Usually this balance can be obtained with good programs of:
Product simplification.
Product standardization.
■■ Product specialization.
■■
■■
Simplification
Simplification is the process of making something easier to do or make. It seeks to cut
out waste by getting rid of needless product varieties, sizes, and types. The emphasis is not
in cutting out products simply to reduce variety but to remove unnecessary products and
variations.
As well as reducing the variety of parts, product design can often be simplified to
reduce operations and material costs. For example, the use of a snap-on plastic cap instead
of a screw cap reduces the cost of both materials and labor.
Standardization
In product design, a standard is a carefully established specification covering the product’s material, configuration, measurements, and so on. Thus, all products made to a given
Products and Processes
343
specification will be alike and interchangeable. Light bulbs are a good example of standardization: the sockets and wattage are standardized and the light bulbs are interchangeable.
A range of standard specifications can be established so it covers most uses for the
item. Men’s shirts are made in a range of standard collar sizes and sleeve lengths so nearly
everyone can be fitted. Most shirt manufacturers also use the same standards so the consumer can get the same size shirt from any manufacturer and expect it to fit.
Because product standardization allows parts to be interchangeable, as long as the
range of standard specifications has been well chosen, a smaller variety of components is
needed. Using the example of light bulbs, the wattages are standardized at 40, 60, and 100
watts. This range allows users to pick wattages that satisfy their needs and manufacturers
to reduce the number of different bulbs, thus reducing inventory.
Another aspect of standardization is the way parts fit together. If the designs of
assemblies are standardized so various models or products are assembled in the same way,
then mass production is possible. The automotive industry designs automobiles so many
different models can be assembled on the same assembly line. For example, several different engines can be mounted in a chassis because the engines are mounted in the same way
and designed so they will all fit into the engine compartment.
Modularization Modularization uses standardized parts for flexibility and variety.
Standardization does not necessarily reduce the range of choice for the customer. By
standardizing on component parts, a manufacturer can make a variety of finished goods,
one of which will probably satisfy the customer. Automobile manufacturing is a prime
example of this. Cars are usually made from a few standard components and a series of
standard options so the consumer has a selection from which to choose. For example, the
Mazda Miata contains 80% parts standard to other Mazda cars, which enables Mazda to
produce the car quickly and at low cost, thus making a profit even though sales are comparatively small. Chrysler uses one platform, the basic frame of the vehicle, for all models
of its minivan, so it has only one set of frame costs for all minivans.
Modularization also permits the practice of postponement, as introduced in Chapter 1,
where the components to manufacture the final product for the customer are made ahead of
time. The final product is then configured to customer specifications once the order is placed.
This reduces the lead time to the customer without the need for final product inventory.
Specialization
Specialization is concentration of effort in a particular area or occupation. Electricians,
doctors, and lawyers specialize in their chosen fields. In product specialization, a firm
may produce and market only one or a limited range of similar products. This leads to process and labor specialization, which increases productivity and decreases costs.
With a limited range of products, productivity can be increased and costs reduced by:
Allowing the development of machinery and equipment specially designed to make
the limited range of products quickly and cheaply.
■■ Reducing the number of setups because of fewer task changes.
■■ Allowing labor to develop speed and dexterity because of fewer task changes.
■■
Specialization is sometimes called focus and can be based either on product and market or on process.
Product and market focus Product and market focus can be based on characteristics such as customer grouping (serving similar customers), demand characteristics
(volume), or degree of customization. For example, one company may specialize in a limited range of high-volume products, whereas another may specialize in providing a wider
range of low-volume products with a high level of customization.
Process focus Process focus is based on the similarity of process. For example, automobile manufacturers specialize in assembling automobiles. Other factories and companies supply the assemblers with components and the assembler specializes in assembly operations.
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a sPEcIal casE: PROcEss INdUsTRIEs
Many products are not produced by discrete production,
and are typically classified as process industries. Some of
the products in this category include chemicals (including
gasoline), paper, glass, and some food products. Some of
these are produced in a process flow production method
but then packaged in a more discrete production mode
(many food products, for example). While the fundamental
concepts of inventory, capacity, scheduling, and so forth
are used in process industries, the specific application of
these can often be different than for discrete production.
As examples, consider that many of the process
industries produce price sensitive commodity products
(even though they can be packaged differently). Often
process operations are designed to run essentially on a
continuous basis, implying major business activities tend
to focus on raw material storage, finished goods storage,
selling the output of the capacity, and on packaging and
transportation scheduling. This is because the fundamental production typically uses specialized equipment
with a narrow product definition, which in some cases is
difficult to shut down and restart. The end of the process
for many products tends to then diverge as the output may
be packaged in several ways and sent to several customers
all over the world. An additional issue that is common for
process companies is the issue of lot tracing for the material produced.
Focused factory Currently there is a trend toward more specialization in manufacturing whereby a factory specializes in a narrow product mix for a niche market. Generally,
focused factories are thought to produce more effectively and economically than more
complex factories, the reason being that repetition and concentration in one area allow the
workforce and management to gain the advantages of specialization. The focused factory
may be a “factory within a factory,” an area in an existing factory set aside to specialize in
a narrow product mix.
Specialization has the disadvantage of inflexibility. Often it is difficult to use highly specialized labor and equipment for tasks other than those for which they were trained or built.
In summary, the three ideas of simplification, standardization, and specialization
are different but interrelated. Simplification is the elimination of the superfluous and is
the first step toward standardization. Standardization is establishing a range of standards
and standard components that will meet most needs. Finally, specialization would not
be possible without standardization. Specialization is concentration in a particular area
and therefore implies repetition, which cannot be arrived at without standard products or
procedures.
A program of product simplification, standardization, and specialization allows a firm
to concentrate on the things it does best, provides the customers with what they want, and
allows operations to perform with a high level of productivity. Reducing part variety will
create savings in raw material, work-in-process, and finished goods inventory. It will also
allow longer production runs, improve quality because there are fewer parts, and improve
opportunities for automation and mechanization. Such a program contributes significantly
to reducing cost.
PROdUcT sPEcIFIcaTION aNd dEsIgN
Product design is responsible for producing a set of specifications that manufacturing can
use to make the product. Products should be designed to be:
Functional.
Capable of low-cost processing.
■■ Environmentally sensitive.
■■
■■
Functional The product will be designed to perform as specified in the marketplace.
The marketing department produces a market specification laying down the expected
performance, sales volume, selling price, and appearance values of the product. Product
design engineers design the product to meet the market specifications. Engineers establish
the dimensions, configurations, and specifications so the item, if properly manufactured,
will perform as expected in the marketplace.
Low-cost processing The product must be designed so it can be made at least cost.
The product designer specifies materials, tolerances, basic shapes, methods of assembling
Products and Processes
345
parts, and so on and, through these specifications, sets the minimum product cost. Usually,
many different designs will satisfy functional and appearance specifications. The job then
is to pick the design that will also minimize manufacturing cost.
Poor design can add cost to processing in several ways:
The product and its components may not be designed to be made using the most economical methods impossible.
■■ Parts may be designed so excessive material has to be removed.
■■ Parts may be designed so operations are difficult.
■■ Lack of standardized components may mean batches of work have to be small. Using
standard parts across a range of products reduces the number of parts in inventory,
tooling, and operator training and permits the use of special-purpose machinery. All
this reduces product cost.
■■ Finally, product design can influence indirect costs such as production planning, purchasing, inventory management, and inspection. For example, one design may call
for twenty different nonstandard parts, whereas another uses fifteen standard parts.
The effort required to plan and control the flow of materials and the operations will
be greater, and more costly, in the first case than the second, due to additional risks of
shortages and dealing with many parts and suppliers.
■■
Environmental or “green” sensitivity When designing products, there are several
environmental issues that should be included or considered. One of these issues concerns
the materials and processes used. Similar to reverse logistics, consideration should be
given as to whether materials or packaging can be:
Reduced to efficiently use resources.
Designed to reduce consumption of energy during the manufacturing process.
■■ Easily separated for reuse.
■■ Recycled.
■■
■■
If not, the company should question if they can make alternative choices that could
be less harmful to the environment. Many of these environmental concerns come under
the overall term sustainability. As discussed in Chapter 2, sustainability includes issues
of being a responsible community “citizen” and being ethical in the approach to doing
business.
Simultaneous Engineering
To design products for low-cost manufacture requires close coordination between product
design and process design, which is called simultaneous or concurrent engineering. If
the two groups can work together, they have a better chance of designing a product that
will function well in the marketplace and can be manufactured at least cost. This relationship between product design and process design can determine the success or failure of a
product. If a product cannot be produced at a cost that will allow a profit to be made, then
it is a failure for the firm.
The traditional approach to product and process design has been a little like a relay
race. When the product design was finished, the work would move to process design and
that department would figure out how to make it. This system has proved time consuming
and expensive and leads to less efficient outcomes. Figure 14.2 shows, with some humor,
what can happen without strong communication and interaction between all parties in the
product development cycle.
Today, many organizations concurrently develop the design for the product and the
processes used to make it. Often a team is made up of people from product design, process design, quality assurance, production planning and inventory control, manufacturing,
purchasing, marketing, field service, and others who contribute to, or are affected by, the
delivery and use of the product to the customer. These groups work together to develop
the product design so it meets the needs of the customer and can be made, delivered to the
customer, and serviced cost-effectively.
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HERE'S WHAT THE
MARKETING FOLKS
REQUESTED
HERE'S WHAT
THE R AND D
BOYS TESTED
HERE'S WHAT THE
CAD CODE DREW
REAL FAST
TENSILE
STRENGTH
2'
HERE'S WHAT WAS
MANUFACTURED IN
MASS
HERE'S WHAT
SALES INSTALLED,
UNDAUNTED
HERE'S WHAT THE
CUSTOMER
REALLY WANTED
Figure 14.2 Communication is essential.
There are several advantages to this approach:
Time to market is reduced. The organization that gets its products to market before
the competition gains a strong competitive advantage.
■■ Cost is reduced. Involving all stakeholders early in the process means less need for
costly product or design changes later.
■■ Better quality. Because the product is designed for ease of production and ease of implementing quality during manufacturing, the number of rejects will be reduced. Because
quality is improved, the need for after-sales service and warranty costs are reduced.
■■ Lower total system cost. Because all groups affected by the product design are consulted, all concerns are addressed. For example, field service might need a product that
is designed so it is easy to service in the field, thus reducing servicing costs.
■■
Supply Chain Collaboration
As part of simultaneous or concurrent engineering, collaboration with suppliers and
customers across the product supply chain can be instrumental in designing a high quality, low cost, and successful product or service. This can be done informally through
conversations with all entities, or more formally through the use of voice of the customer (VOC), or utilizing quality function deployment (QFD). This will be discussed
further in Chapter 16. In addition to the results already mentioned for simultaneous
engineering, customer satisfaction is improved by aligning products with customer
needs, and the service or product features is perceived as value-added.
PROcEss dEsIgN
Operations management is responsible for producing the products and services the customer wants, when wanted, with the required quality, at a desired cost, and with high
effectiveness and productivity. Processes are the means by which operations management
reaches these objectives.
Products and Processes
347
LEVEL ZERO
LEVEL ONE
Figure 14.3 Nesting concept.
A process is a method of doing something, generally involving a number of steps or
operations. Process design is the developing and designing of the steps.
Every activity involves a process of some type. Going to the bank to deposit or
withdraw money, preparing a meal, or going on a trip involves a process or series of processes. Sometimes consumers are personally involved in the process. Most have waited
at a checkout counter in a store and wondered why management has not devised a better
process for serving customers.
Nesting Another way of looking at the hierarchy of processes is the concept of nesting.
Several small processes are linked to form a larger process. Consider Figure 14.3. Level
zero shows a series of steps, each of which may have its own series of steps. One of the
operations on level zero is expanded into its component parts and shown on level one. The
nesting can continue to further levels of detail.
Mass customization Recent changes in process flexibility have allowed for the development of a concept called mass customization. If the operation is designed to be flexible and efficient enough, it will allow the production of customized products (specific to
customer demand) at virtually the same cost as mass-produced product. Customization in
general requires the ability to quickly redesign and produce a product or service based on
customer need. In some cases, the customization can occur at the final stage, referred to as
postponement, discussed in Chapter 1. Whatever type of customization is incorporated,
the key is to design a product and process that combines flexibility, agility, and knowledge
of customer needs.
FacTORs INFlUENcINg PROcEss dEsIgN
Six basic factors must be considered when designing a process.
Product design and quality level The product’s design determines the basic processes needed to convert the raw materials and components into the finished product. For
example, if a steak is to be barbecued, then the process must include a barbecue operation.
The process designer can usually select from a variety of different machines or operations
to do the job. The type of machine or operation selected will depend upon the quantity to
be produced, the available equipment, and the quality level needed.
The desired quality level affects the process design because the process must be
capable of achieving that quality level and doing it repeatedly. If the process cannot do
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that, operations will not be able to produce what is wanted except with expensive inspection and rework. The process designer must be aware of the capabilities of machines and
processes and select those that will meet the quality level at least cost.
Demand patterns and flexibility needed If there is variation in demand for a
product, the process must be flexible enough to respond to these changes quickly. For
example, if a full-service restaurant sells a variety of foods, the process must be flexible
enough to switch from broiling hamburgers to making pizzas. Conversely, if a pizza parlor
sells nothing but pizzas, the process need not be designed to cook any other type of food.
Flexible processes require flexible equipment and personnel capable of doing a number of
different jobs.
Quantity/capacity considerations Product design, the quantity to produce, and process design are closely related. Both product and process design depend on the quantity
needed. For example, if only one of an item is to be made, the design and the process used
will be different than if the volume is 100,000 units. The quantity needed and the process
design determine the capacity needed. Figure 14.4 shows this relationship. Note that all
three are directly connected to the customer.
Customer involvement Chapter 1 discussed five manufacturing strategies—engineer-to-order, make-to-order, configure-to-order, assemble-to-order, and make-to-stock—
and the extent of customer involvement in each. Process design will depend on which
strategy is chosen.
Environmental concerns Just as in product design, the process design should have
minimal impact on the environment, if at all possible. In addition, concern should be
taken to minimize the amount of energy utilized. This consideration also directly impacts
the cost of running the process. Painting often involves the use of solvents and other
chemicals that are harmful to the environment and workers. Water-based paints have been
developed that reduce emissions and cleanup. The paint and the equipment used to apply
it is often expensive but the company saves in cleanup and worker protection costs.
Make or buy decision A manufacturer has the alternative of making parts in-house or
of buying them from an outside supplier. Few companies make everything or buy everything they need. Indeed, on the average, North American manufacturers purchase more
than 50% of the cost of goods manufactured. A decision has to be made about which items
to make and which to buy. Although cost is the main determinant, other factors such as the
following are usually considered:
What to
produce?
PRODUCT
DESIGN
CUSTOMER
PROCESS
DESIGN
How to
produce?
CAPACITY
How much
to produce?
Figure 14.4 Product design, process design, and capacity are closely related.
Products and Processes
349
Reasons to Make In-house
Produces for less cost than a supplier.
■■ Utilizes existing equipment to fullest extent.
■■ Keeps confidential processes within control of the firm.
■■ Maintains quality.
■■ Maintains workforce.
■■
Reasons to Buy
Requires less capital investment.
■■ Uses specialized expertise of suppliers.
■■ Allows the firm to concentrate on its own area of specialization.
■■ Provides known and competitive prices.
■■ Accommodates large changes in volume.
■■
The decision to make or buy is clear for many items such as nuts and bolts, motors,
or components that the firm does not normally manufacture. For other items that are in the
firm’s specialty area, a specific decision will have to be made.
As supply chains are becoming more tightly linked and sources of supply are becoming more global, the issue to buy (outsource) or make (insource) becomes more complex.
For example, exchange rates, transit inventory levels, impact of transit time on lead time,
and government controls are all becoming issues that need to be considered in making the
decision to make or buy.
PROcEssINg EqUIPmENT
Processing equipment can be classified in several ways. This discussion will focus on
classification by the degree of specialization of machinery and equipment.
General-purpose machinery General-purpose machinery can be used for a variety of operations or can do work on a variety of products within its machine classification.
For example, a home sewing machine can sew a variety of materials, stitches, and patterns
within its basic capability. Different auxiliary tools can be used to create other stitches or
for particular sewing operations.
Special-purpose machinery Special-purpose machinery is designed to perform
specific operations on one work piece or a small number of similar work pieces. For
example, a sewing machine built or equipped to sew shirt collars would be a specialpurpose machine capable of sewing collars on any size shirt of any color but not capable
of performing other sewing operations unless it was modified extensively.
General-purpose machinery is generally less costly than special-purpose machinery.
However, its run time can be slower, and because its operations often require more human
input, the quality level tends to be more variable than when using special-purpose machinery. One exception is robotic machines, which automate the human aspect and tend to
produce high quality on repetitive processes, but the cost is much higher. Special-purpose
machinery is less flexible but parts can generally be produced with it much quicker than
with general-purpose machinery.
PROcEss sysTEms
Depending on the product design, volume, and available equipment, the process engineer
must design the system to make the product. As mentioned in Chapter 6, based on material
flow, processes can be organized in three ways:
Flow.
Intermittent.
■■ Project.
■■
■■
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The system used will depend on the demand for the item, range of products, and the
ease or difficulty of moving material. All three systems can be used to make discrete units
such as automobiles or textbooks, or to make nondiscrete process products such as gasoline, paint, or fertilizer.
Flow Processes
Workstations needed to make the product, or family of similar products, are grouped
together in one department and are laid out in the sequence needed to make the product.
Examples are assembly lines, cafeterias, oil refineries, and steel rolling mills. In flow
processing, work flows from one workstation to another at a nearly constant rate and with
no delays. Continuous flow processing is used to manufacture products such as liquids,
basic metals, and commodities such as petroleum products. There is some method of
moving goods between workstations. If the units are discrete, such as automobiles, flow
manufacturing is called repetitive manufacturing. The typical flow pattern is shown in
Figure 14.5. Flow process layout is sometimes called product layout because the system
is set up for a limited range of similar products.
Flow systems produce only a limited range of similar products. For example, an
assembly line that produces a certain type of refrigerator cannot be used to assemble
washing machines. The operations and materials used to make one are different and in a
different sequence than those used for making the other. Demand for the family of products has to be large enough to justify setting up the line economically. If sufficient demand
exists, flow systems are extremely efficient, for several reasons:
Workstations are designed to produce a limited range of similar products, so machinery and tooling can be specialized.
■■ Because material flows from one workstation to the next, there is very little buildup of
work-in-process inventory.
■■ Because of the flow system and the low work-in-process inventory, lead times are short.
■■ In most cases, flow systems substitute capital for labor and standardize what labor
there is into routine tasks.
■■
Intermittent Processes
In intermittent manufacturing, goods are not made continuously as in a flow system but
are made at intervals in lots or batches. Workstations must be capable of processing many
different parts. Thus, it is necessary to use general-purpose workstations and machinery
that can perform a variety of tasks.
General-purpose workstations do not produce goods as quickly as special-purpose
workstations used in flow manufacturing. Usually, workstations are organized into departments based on similar types of skills or equipment. For example, all welding and fabrication operations are located in one department, machine tools in another, and assembly
in yet another department. Work moves only to those workstations needed to make the
product and skips the rest. This results in the jumbled flow pattern shown in Figure 14.6.
This is referred to as a process layout, or functional layout, in which similar functions
or equipment are grouped together, and product flows from work center to work center.
Intermittent processes are flexible. They can change from one part or task to another
more quickly than can flow processes. This is because they use general-purpose machinery and skilled flexible labor that can perform the variety of operations needed.
WORKSTATIONS
Output
Input
1
2
Figure 14.5 Material flow: flow process.
3
4
Products and Processes
351
WORKSTATIONS
Product A
Product B
Product C
1
2
3
4
5
6
7
8
9
Figure 14.6 Material flow: intermittent process.
Control of work flow is managed through individual work orders for each lot or
batch being made. Because of this and the jumbled pattern of work flow, manufacturing
planning and control of the shop floor are critical. Often, many work orders exist, each of
which can be processed in different ways.
Provided the volume of work exists to justify it, flow manufacturing is less costly
than intermittent manufacturing. There are several reasons for this:
Setup costs are low. Once the line is established, changeovers are needed infrequently
to run another product.
■■ Since work centers are designed for specific products, run costs are low.
■■ Because products move continuously from one workstation to the next, work-in-process inventory will be low.
■■ Costs associated with controlling production are low because work flows through the
process in a fixed sequence.
■■
But the volume of specific parts must be enough to use the capacity of the line and
justify the capital cost.
Project Processes
Project manufacturing is mostly used for large, complex projects such as locomotives,
ships, or buildings, and is typically a one-time endeavor, used to create a unique product
or service. The product may remain in one location for its full assembly period known as
a fixed position layout, as with a ship, or it may move from location to location after considerable work and time are spent on it. Project manufacturing avoids the very high costs
of moving the product from one workstation to another.
There are many variations and combinations of these three basic types of processes.
Companies try to find the best combination to make their particular products. In any one
company it is not unusual to see examples of all three being used.
PROcEss cOsTINg
There are two common methods for determining product costs. Job costing is used when
multiple products are produced within a period, and costs such as labor, material, and
overhead are allocated to each product. The other method is process costing, which is
most commonly used in industries that manufacture product in a continuous process, such
as paper, petroleum, or concrete. In these process industries, it is impossible to allocate
specific costs for a period to a specific lot, as the flow is continuous. Materials, labor,
and overhead consumed during a particular period are accumulated, and then allocated
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to departments or operations, rather than a specific product. Overall product cost can be
determined by summing the accumulated costs, and then dividing that by the volume of
products produced during the time period.
sElEcTINg ThE PROcEss
Generally, the larger the volume (quantity) to be produced, the greater the opportunity to use
special-purpose processes. The more special purpose an operation, the faster it will produce.
Often the capital cost for such machinery or for special tools or fixtures is high. Capital costs
are called fixed costs and the production, or run, costs are called variable costs.
Fixed costs These are costs that do not vary with the volume being produced. Purchase
costs of machinery and tools and setup costs are considered fixed costs. No matter what
volume is produced, these costs remain the same. Suppose it costs $200 to set up a process; this cost will not change no matter how much is produced.
Variable costs These are costs that vary with the quantity produced. Direct labor
(labor used directly in the making of the product) and direct material (material used
directly in the product) are the major variable costs. If the runtime for a product is 12 minutes per unit, the labor cost $10 per hour, and the material cost $5 per unit, then:
Variable cost =
12
* $10.00 + $5.00 = $7.00 per unit
60
Let:
FC = Fixed cost
VC = Variable cost per unit
x = Number of units to be produced
TC = Total cost
UC = Unit 1average2 cost per unit
Total cost = Fixed cost + 1Variable cost per unit21number of units produced2
Then:
TC = FC + VCx
Unit cost =
Total cost
TC
=
x
number of units produced
example Problem
A process designer has a choice of two methods for making an item. Method A has a
fixed cost of $2000 for tooling and jigs and a variable cost of $3 per unit. Method B
requires a special machine costing $20,000 and the variable costs are $1 per piece.
Let x be the number of units produced.
Fixed cost
Variable unit cost
Method A
Method B
$2000.00
$20,000.00
$3.00
$1.00
Total cost
$2000.00 + 3x
$20,000.00 + 1x
Unit (average) cost
$2000.00 + 3x
x
$2000.00 + 1x
x
Table 14.1 shows what happens to the total cost as quantities produced are increased.
The total cost data in this table is shown graphically in Figure 14.7. From Table 14.1 and
Figure 14.7, we can see that initially the total cost and unit cost of method A are less than
method B. This is because the fixed cost for method B is greater and has to be absorbed over
Products and Processes
Volume
Total Cost (Dollars)
353
Unit Cost (Dollars)
(Units)
Method A
Method B
Method A
Method B
2000
$8,000
$22,000
$4.00
$11.00
4000
14,000
24,000
3.5
6
6000
20,000
26,000
3.33
4.33
8000
26,000
28,000
3.25
3.5
10,000
32,000
30,000
3.2
3
12,000
38,000
32,000
3.17
2.67
14,000
44,000
34,000
3.14
2.43
16,000
50,000
36,000
3.13
2.25
Table 14.1
Total and average cost versus quantity produced.
50
LEGEND
METHOD A
METHOD B
COST ($1000)
40
30
20
COST
EQUALIZATION
POINT
10
0
0
2
4
6
8
10
12
14
16
VOLUME (1000 UNITS)
Figure 14.7 Total cost versus quantity produced.
a small number of units. Although the total cost for both methods increases as more units are
produced, the total cost for method A increases faster until, at some quantity between 8000
and 10,000 units, the total cost for method B becomes less than for method A.
Similarly, the unit cost for both methods decreases as more units are produced.
However, the unit cost for method B decreases at a faster rate until, at some quantity
between 8000 and 10,000 units, unit costs for both methods are equal.
Cost Equalization Point
Knowing the quantity beyond which the cost of using method B becomes less than for method
A enables the decision of which process to use to minimize the total cost (and the unit cost).
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Chapter Fourteen
This quantity is called the cost equalization point (CEP) and is the volume for
which the total cost (and unit cost) of using one method is the same as another. For the
example, the total cost calculations are as follows:
TCA = TCB
FCA + VCAx = FCB + VCBx
$2000 + $3x = $20,000 + $1x
3x - 1x = 20,000 - 2000
2x = 18,000
x = 9000 units
The CEP is 9000 units. At this quantity, the total cost of using method A will be the
same as for method B.
TCA = FCA + VCAx - 2000 + 13 * 90002 = $29,000
TCB = FCB + VCBx = 20,000 + 11 * 90002 = $29,000
We can get the same results by using unit costs instead of total costs.
From the preceding calculations it is clear that:
If the volume (quantity to produce) is less than the CEP, the method with the lower
fixed cost will cost less.
■■ If the volume is greater than the CEP, the method with the greater fixed cost will cost less.
■■
For example, if the volume were 5000 units, method A would cost less, and if the
volume were 10,000 units, method B would cost less.
Variable costs, mostly direct labor and material, can be reduced by substituting
machinery and equipment (capital) for direct labor. This increases the fixed costs and
decreases the variable costs. But to justify this economically, the volume must be high
enough to reduce the total or unit cost of production.
Increasing Volume
The obvious way to increase volume is to increase sales. However, a finished product is
usually made up of several purchased or manufactured components. If the volume of these
components can be increased, then the unit cost of the components, and the final product,
will be reduced.
The volume of components can be increased without increasing sales by a program of
simplification and standardization, discussed previously in this chapter.
If a subassembly or component part can be standardized for use in more than one final
product, then the volume of the subassembly or part is increased without an increase in the
total sales volume. Thus, more specialized and faster-running processes can be justified
and the cost of operations reduced.
Standardization of parts is a major characteristic of modern mass production. At the
turn of the twentieth century, Henry Ford revolutionized manufacturing by standardizing
the finished product—one model of car. The joke often heard then was that you could
have any color you wanted as long as it was black. Today a vast range of models are made,
but if each model was exploded into its subassemblies and component parts, one would
find specific components common to a great number of models. In this way, modern
manufacturers can provide the consumer with a wide choice of finished products made
from standard parts and components.
cONTINUOUs PROcEss ImPROvEmENT
People have always been concerned with how best to do a job and the time it should take
to do it. Process improvement is concerned with improving the effective use of human and
other resources. Continuous implies an ongoing activity; improvement implies an increase
in the productivity or value of quality or condition. Hence, the name continuous process
improvement (CPI).
Products and Processes
355
Continuous process improvement consists of a logical set of steps and techniques
used to analyze processes and to improve them.
Improving productivity Productivity can be improved by spending money (capital)
on better and faster machines and equipment. However, with any given amount of capital, a method must be designed to use the machinery and equipment most productively.
A workstation might consist of highly sophisticated machinery and equipment worth $1
million or more. Its productivity and return on investment depend on how the equipment
is used and how the operator manages it.
Continuous process improvement is a low-cost method of designing or improving
work methods to maximize productivity. The aim is to increase productivity by better use
of existing resources. Continuous process improvement is concerned with removing work
content, not with spending money on better and faster machines.
Peter Drucker has said, “Efficiency is doing things right; effectiveness is doing the
right things.” CPI aims to do the right things and to do them efficiently.
People involvement Today management recognizes the need to maximize the potential of flexible, motivated workers. People are capable of thinking, learning, problem
solving, and contributing to productivity. With existing processes and equipment, people
are the primary source of improvement because they are the experts in the things they do.
Process improvement is not solely the responsibility of industrial engineers. Everyone
in the workforce must be given the opportunity to improve the processes they work with.
Techniques that help to analyze and improve work are not complicated and can be learned.
Indeed, the idea of continuous improvement is based on the participation of operators and
improvement in methods requiring relatively little capital.
Workers have two jobs:
■■
■■
Their “as defined” job.
To improve their “as defined” job.
Teams One of the features of CPI is team involvement. A team is a group of people
working together to achieve common goals and objectives. The members of the team
should be all those who are involved with the process. Teams are successful because of
the emphasis placed on people. Not all problems can be solved by teams, nor are all people suited to teams. However, they are often effective, as problems often cross functional
lines and thus multifunctional teams are common.
Continuous process improvement can still be effectively carried out by the individuals.
The Six Steps in Continuous Process Improvement
The general method used to solve many kinds of problems with CPI includes six steps:
1. Select the process to be studied.
2. Record the existing method to collect the necessary data in a useful form.
3. Analyze the recorded data to generate alternative improved methods.
4. Develop the best method of doing the work by evaluating the alternatives.
5. Implement the method as standard practice by training the operator.
6. Maintain the new method.
Select the Process
The first step is to decide what to study. This depends on the ability to recognize situations
that have good potential for improvement. Observation of existing methods comes first.
Observe The important feature in observation is a questioning attitude. Questions such
as why, when, and how must be asked whenever something is observed. This attitude
needs development because people tend to assume that the familiar method is the only
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Chapter Fourteen
one. Often heard is, “We have always done it this way!” However, “this way” is not necessarily the only, most productive, or most effective way.
Any situation can be improved but some have better potential than others. Indicators
in manufacturing that show areas most needing improvement include:
High scrap, reprocessing, rework, and repair costs.
Backtracking of material flow caused by poor plant layout.
■■ Bottlenecks.
■■ Excessive overtime.
■■ Excessive manual handling of materials, both from workplace to workplace and at the
workplace.
■■ Use of environmentally hazardous materials.
■■ Employee grievances without true assignable causes.
■■
■■
Select The purpose of continuous process improvement is to improve productivity to
reduce operating, product, or service costs. In selecting jobs or operations for method
improvement, there are two major considerations: economic and human.
Economic considerations The cost of the improvement must be justified. The cost of
doing the study and implementing the improvement must be recovered from the savings in
a reasonable time. One to two years is a commonly used period.
The job size must justify the study. Almost anything can be improved, but the
improvement must be worthwhile. Suppose a process improvement saves 1 hour on a
job taking 5 hours, performed once a month or 12 times a year. The reduction in time is
20% and the total time saved in a year is 12 hours. Another process improvement saves 1
minute on a job taking 10 minutes, performed 200 times per week. The time saved in this
case is only 10% but will be 173 11 * 200 * 52 , 60 = 173.32 hours per year, a much
higher rate of return on the investment made in the study.
The human factor The human factor governs the success of continuous improvement.
The resistance to change, by both management and worker, must always be remembered.
Working situations characterized by high fatigue, accident hazards, absenteeism, and dirty
and unpleasant conditions should be identified and improved. Sometimes it is difficult to
give specific economic justification for such improvements, but the intangible benefits are
extensive and should weigh heavily in selecting studies.
Pareto diagrams Pareto analysis can be used to select problems with the greatest
economic impact. The theory of Pareto analysis is the same as that used in the ABC analysis discussed in Chapter 9. This theory says that a few items (usually about 20%) account
for most of the cost or problems. It separates the “vital few” from the “trivial many.”
Examples of the “vital few” are as follows:
A few processes account for the bulk of scrap.
A few suppliers account for most rejected parts.
■■ A few problems account for most process downtime.
■■
■■
The steps in performing a Pareto analysis are as follows:
1. Determine the method of classifying the data: by problem, cause, nonconformity, and
so forth.
2. Select the unit of measure. This is usually dollars but may be the frequency of
occurrence.
3. Collect data for an appropriate time interval, usually long enough to include all likely
conditions.
4. Summarize the data by ranking the items in descending order according to the
selected unit of measure.
5. Calculate the total cost.
Products and Processes
357
6. Calculate the percentage for each item.
7. Construct a bar graph showing the percentage for each item and a line graph of the
cumulative percentage.
example Problem
A product has failed in the field a number of times. Data is collected according to
the type of failure with the following results: Type A—11, type B—8, type C—5, type
D—60, type E—100, type F—4, other—12. Construct a table summarizing the data
in descending order of magnitude. From this table, construct a Pareto diagram.
Answer
Type of
Failure
Number of
Failures
Percent
Cumulative
Percentage
E
100
50.0
50.0
D
60
30.0
80.0
A
11
5.5
85.5
B
8
4.0
89.5
C
5
2.5
92.0
F
4
2.0
94.0
O (Other)
12
6.0
100.0
Total
200
100.0
TYPES OF FIELD FAILURES
100
100
PERCENT FREQUENCY
85.5
92
89.5
94
80
80
60
50
40
30
20
5.5
0
E
D
4
2.5
2
A
B
C
TYPE OF FAILURE
F
6
O
Note that Pareto analysis does not report what the problems are, only where they
seem to occur. In the previous example, further investigation into the causes of failure
types E and D will give the best return for the effort spent. It is important to select the
categories carefully. For example, in the previous problem, if the location of failures were
recorded rather than the type of failure, the results would be quite different and perhaps
not significant.
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Chapter Fourteen
PEOPLE
MACHINES
MATERIALS
PROBLEM TO
BE SOLVED
ENVIRONMENT
MEASUREMENTS
METHODS
Figure 14.8 Cause-and-effect diagram.
Cause-and-effect diagram Sometimes called a fishbone or Ishikawa diagram, the
cause-and-effect diagram is a very useful tool for identifying root causes. Figure 14.8
shows such a diagram.
The fishbone diagram is best used by a group or team working together. It can be
constructed by discussion and brainstorming. The steps in developing a fishbone diagram
are as follows:
1. Identify the problem to be studied and state it in a few words. For example, the reject
rate on machine A is 20%.
2. Generate some ideas about the main causes of the problem. Usually all probable root
causes can be classified into six categories.
■■ Materials. For example, from consistent to inconsistent raw materials.
■■ Machines. For example, a well-maintained machine versus a poorly maintained
one.
■■ People. For example, a poorly trained operator instead of a well-trained one.
■■ Methods. For example, changing the speed on a machine.
■■ Measurement. For example, measuring parts with an inaccurate gauge.
■■ Environment. For example, increased dust or humidity.
3. Brainstorm all possible causes for each of the main causes.
4. Once all the causes have been listed, try to identify the most likely root causes and
work on these.
Record
The next step is to record all the facts relating to the existing process. To be able to
understand what to record, it is necessary to define the process being studied. Recording
defines the process. The following must be determined to properly define the process.
The process boundaries. All processes, big or small, begin and end somewhere.
Starting and ending points form the boundaries of the process. For example, the starting point in the process of getting to work in the morning might be getting out of bed.
The ending point might be arrival at the desk or classroom.
■■ Process flow. This is a description of what happens between the starting and ending
points. Usually this is a listing of the steps taken between the start and finish of the
process. There are several recording techniques to help perform this step. Some of
these will be discussed later in this chapter.
■■ Process inputs and outputs. All processes change something. The things that
are changed are called inputs and they may be physical, such as raw materials, or
■■
Products and Processes
359
informational, such as data. Outputs are the result of what goes on in the process. For
example, raw materials are converted into something more useful or data is manipulated to produce reports.
■■ Components. Components are the resources used in changing inputs to outputs. They
are composed of people, methods, and equipment. Unlike process inputs, components
do not become part of the output but are part of the process. For example, in producing
a report, the graphics program, computer, and printer are all components.
■■ Customer. Processes exist to serve customers and customers ultimately define what
a process is supposed to do. If customer needs are not considered, there is a risk of
improving things that do not matter to the users of the output.
■■ Suppliers. Suppliers are those who provide the inputs. They may be internal to the
organization or external.
■■ Business environment. The process is controlled or regulated by external and internal factors. The external factors are beyond the firm’s control and include customers’
acceptance of the process output, competitors, and government regulation. Internal
factors are within the organization and can be controlled.
Figure 14.9 shows a schematic of the process.
The next step is to record all facts relating to the existing method. A record is necessary because it is difficult to record and maintain a large mass of detail by memory for the
duration of the analysis. Recording helps the team consider all elements of the problem in
a logical sequence and makes sure all the steps in the process are considered. The record
of the present method also provides the basis for both the critical examination and the
development of an improved method.
Classes of activity Before discussing some of the charts used, the kinds of activities
recorded will be discussed. All activity can generally be classified into one of six types.
As a method of shorthand, there are six universally used symbols for these activities. The
activities and symbols are shown in Figure 14.10.
Following are descriptions of some of the various charting techniques.
Operations process charts Operations process charts record in sequence only the
main operation and inspections. They are useful for preliminary investigation and give a
bird’s-eye view of the process. Figure 14.11 shows such a chart.
The description, and sometimes the times, for each operation is also shown. An operations process chart would be used to record product movement.
Process flow diagram A process flow diagram shows graphically and sequentially
the various steps, events, and operations that make up a process. It provides a picture, in
the form of a diagram, of what actually happens when a product is made or a service performed. In addition to the six symbols shown in Figure 14.10, others may be used to show
ENVIRONMENT
•Internal
•External
INPUT
(Suppliers)
PROCESS
(Work Steps)
COMPONENTS
•People
•Methods
•Equipment
Figure 14.9 Schematic of a process.
OUTPUT
(Customers)
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Chapter Fourteen
Operation. The main step in a process, method, or procedure.
Usually the part, material, or product is modified during the
operation.
Inspection. An inspection for quality or a check for quantity.
Where process can be measured, regulated, and controlled.
Movement. The movement of workers, material, equipment, or
information from place to place.
Storage. A controlled storage from which material is issued or
received.
Delay. A delay in the sequence of events; for example, material
waiting to be worked on.
Decision. Where a decision is made.
Figure 14.10 Classes of activity.
PART "B"
PART "A"
MAIN COMPONENT
TIME
5
TIME
OPERATION
DESCRIPTION
3
OPERATION
TIME
DESCRIPTION
1
OPERATION
DESCRIPTION
TIME
6
TIME
OPERATION
DESCRIPTION
4
OPERATION
TIME
DESCRIPTION
2
OPERATION
DESCRIPTION
TIME
7
OPERATION
DESCRIPTION
INSPECTION
TIME
8
OPERATION
DESCRIPTION
TIME
9
OPERATION
DESCRIPTION
PURCHASED PART
INSPECTION
Figure 14.11 Operations process chart.
Products and Processes
361
FILE UNTIL
INVOICE AND
PURCHASE
ORDER
RECEIVED
NO
APPROVE
FOR
PAYMENT
YES
RECEIVING
REPORT
RECEIVED
INVOICE AND
PURCHASE
ORDER ON
FILE?
VERIFY
INVOICE
MAIL
CHECK
SEND TO
ACCOUNTS
PAYABLE
PREPARE
CHECK
Figure 14.12 Process flow diagram.
such things as rework and documentation. Figure 14.12 shows an example of a process
flow diagram. In this example, the process starts when the goods are received and ends
when a check is sent to the supplier.
Analyze
Examination and analysis are the key steps in continuous process improvement. Although
all the other steps are significant, they either lead up to, or result from, the critical analysis.
This step involves analyzing every aspect of the present method and evaluating all proposed possible methods.
Find the root cause Frequently it is difficult to separate symptoms from the root
causes of problems. Often the only thing observed are symptoms and it is difficult to trace
back to the root cause. To find root causes requires a questioning attitude. For the analyst,
“why” is the most important word. Every aspect of the existing method should be questioned. In “The Elephant’s Child,” from The Just-So Stories, Rudyard Kipling wrote:
I keep six honest serving-men
(They taught me all I know);
Their names are What and Why and When
And How and Where and Who.
Kipling personifies six words normally used as questions; they are also the “serving
men” of the methods analyst.
A rule of thumb common to many problem-solving methods says it is necessary to ask
(and answer) the “why” question up to five times before one reaches the root cause of a problem. As each “why” is answered, the question is asked of “why” that was the correct answer.
Three approaches can help in examining:
A questioning attitude. This implies an open mind, examining the facts as they are,
not as they seem, avoiding preconceived ideas, and avoiding hasty judgments.
■■ Examining the total process to define what is accomplished, how, and why. The
answers to these questions will determine the effectiveness of the total process. The
results may show that the process is not even needed.
■■
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Chapter Fourteen
Examining the parts of the process. Activities can be divided into two major categories: those in which something is happening to the product (worked on, inspected, or
moved) and those in which nothing constructive is happening to the product (delay or
storage, for example). In the first category, value is added only when the part is being
worked on. Setup, put away, and move, while necessary, add cost to the product but do
not increase its value, so must be minimized. Value will be added when the product is
being worked on but, again, the goal is to maximize the productivity of these operations.
■■ Analysis of the relationship between production rate, item throughput, and
process inventory. For processes that have the rate of material input equal to the rate
of output, there is a basic relationship between the amount of inventory in the process
and the time it takes for a single item to be processed through the process (throughput
time). If the processing rate of the process is called R, the inventory is called I, and the
throughput time is called T, then the relationship is given as
■■
I = RT
example Problem
A process has the ability to produce two items every minute. The process currently has
100 of the items being processed. If another item were introduced into the process,
how long would it take until the new item is produced?
Answer
The production rate is two items per minute, and the inventory is 100 units. The solution would then be as follows:
I = RT
100 units = (2 units/minute) * T
T = 50 minutes
This relationship is often called Little’s Law, which can be applied to any process
that includes time, inventory, and throughput. For example, to estimate the time a customer waits in line for a service could be computed as
I (# of customers) = R (minutes of service) * T (minutes a customer will wait to be served)
8 customers = 0.5 people per minute * T
T = 16 minutes
Develop
When developing possible solutions, there are four approaches to take to help develop a
better method.
Eliminate all unnecessary work. Question why the work is being done in the first place
and if it can be eliminated.
■■ Combine operations wherever possible. Thus, material handling will be reduced, space
saved, and the throughput time reduced. This is a major thrust of lean manufacturing.
■■ Rearrange the sequence of operations for more effective results. This is an extension of
the previous approach. If sequences are changed, then they can possibly be combined.
■■ Simplify wherever possible by making the necessary operations less complex. If the
questioning attitude is used, then complexity should be reduced. Usually the best
solutions are the simple ones.
■■
Principles of motion economy There are several principles of motion economy,
including the following:
1. Locate materials, tools, and workplace within normal working areas and pre-position
tools and materials.
2. Locate the work done most frequently in the normal working areas and everything
else within the maximum grasp areas.
Products and Processes
363
3. Arrange work so motions of hands, arms, legs, and so on are balanced by being
made simultaneously, in opposite directions, and over symmetrical paths. Both hands
should be working together and should start and finish at the same time. The end of
one cycle should be located near the start of the next cycle.
4. Conditions contributing to operator fatigue must be reduced to a minimum. Provide good
lighting, keep tools and materials within maximum working areas (see Figure 14.13),
provide for alternate sitting and standing at work, and design workplaces of proper
height to eliminate stooping.
Human and environmental factors In addition to the principles of motion economy, other important matters that influence human and environmental conditions must
be considered. These include safety, comfort, cleanliness, and personal care, so provision must be made for lighting, ventilation and heating, noise reduction, seating, and
stimulation.
Of these, stimulation may be the least obvious. In highly repetitive work, workers
may become bored and dissatisfied, which may lead to emotional problems. A pleasant
environment created by attractive color schemes in plants or offices, location of windows,
and music during working hours can do much to reduce stress and absenteeism.
Methods improvement is based on the concepts of scientific management. It concentrates on the task and ways of removing work content (waste) from tasks. It gives little
consideration to the human being and higher-level needs, such as self-esteem and self-fulfillment, and work can become repetitive and boring. Job design is an attempt to provide more
satisfying meaningful jobs and to use the worker’s mental and interpersonal skills. These
improvements include the following:
Job enlargement expands a worker’s job by clustering similar or related tasks into
one job. For example, a job might be expanded to include a sequence of activities instead
of only one activity. This is called horizontal enlargement.
Job enrichment adds more meaningful, satisfying, and fulfilling tasks. The job not
only includes production operations but also many setup, scheduling, maintenance, and
control responsibilities.
Job rotation trains workers to do several jobs so they can be moved from one job to
another. This is called cross-training.
All these factors help to create a more motivated and flexible workforce. In modern
manufacturing, where quick response to customers’ needs is essential, these characteristics can mean the difference between business success and failure.
Employee empowerment and self-directed teams assume the workers have more
knowledge and responsibility for understanding and performing the work. Empowering
the workers to take more responsibility can lead to self-directed teams, where there is typically no supervisor. The workers themselves understand what is to be done and manage
their own activities to accomplish the required work efficiently and effectively with little
or no supervision.
MAXIMUM WORKING AREA
(SHOULDER MOVEMENT)
Figure 14.13 Working areas.
NORMAL WORKING AREA
(FINGER, WRIST, AND
ELBOW MOVEMENTS)
Chapter Fourteen
OUTPUT
364
TIME
Figure 14.14 Learning curve.
Implement
So far, the work done by the analyst has been planning. Now the plans must be put into
action by implementing the new procedure.
In planning the implementation process, consideration must be given to the best time
to implement, the method of implementation, and the people involved. The analyst needs to
be sure that equipment, tooling, information, and the people are all available. At the time of
implementation, a dry run will show whether all equipment and tooling are working properly.
Training the operator is the most important part of the implementation process. If
the operator has been involved in designing the change, this should not be difficult. The
worker will be familiar and comfortable with the change and will probably feel some
sense of ownership.
Learning curve Over time, as the operator does the tasks repetitively, speed will
increase and errors decrease. This process is known as the learning curve and is illustrated
in Figure 14.14. Note there is no time scale shown. Depending on the task, a worker
may progress through the learning curve in a few minutes or, for high-skill jobs, several
months or years.
Maintain
Maintaining is a follow-up activity that has two parts. The first is to be sure that the new
method is being done as it should be. This is most critical for the first few days, and close
supervision may be necessary. The second is to evaluate the change to be sure that the
planned benefits are accomplished. If not, the method must be changed.
sUmmaRy
Products and the processes used to make them are continually being redesigned to create
products more appealing to customers, to improve productivity, or to make the products
and their associated processes friendlier to the environment. Producers follow wellestablished principles of product development. The 3Ss, simplification, standardization,
and specialization, are very powerful principles that can help to improve productivity
and make products more reliable. With information easy to share in today’s environment,
product design and process design can work simultaneously to bring better quality products to market faster and with reduced costs. As the design of the product is being established, the process is also being designed based on: the quality level desired, the ability
Products and Processes
365
of the process to react quickly to changes in customer demand (flexibility), the overall
volume of demand, and how much the customer wants to be involved in the production
of the product. Depending on the volume, the decision may be made to buy the product
rather than make it, or to use one process rather than another. This is best determined with
the cost equalization point (CEP), which determines a volume below which the low fixed
cost, high variable cost alternative is used, and above which a company may invest in the
high fixed cost and low variable cost alternative.
Continuous process improvement (CPI) applies to all processes to make them more
cost effective and competitive. It is done on an ongoing basis, not just when new products
are introduced. CPI uses the traditional tools of the scientific methods in six steps: select,
record, analyze, develop, implement, and maintain. All the steps have one thing in common and that is the involvement of people.
With improvements to product and process, design goods tend to flow quickly and
smoothly, resulting in lower costs and improved profits. Continuous improvement leads to
the next chapter in this text on just-in time-and lean production.
KEy TERms
Cause-and-effect diagram 358
Concurrent engineering 345
Continuous flow processing 350
Continuous process improvement (CPI) 355
Cost equalization point (CEP) 354
Cross-training 363
Employee empowerment 363
Fishbone diagram 358
Fixed costs 352
Fixed position layout 351
Flow processing 350
Focused factories 344
Functional layout 350
General-purpose machinery 349
Intermittent manufacturing 350
Job design 363
Job enlargement 363
Job enrichment 363
Job costing 351
Job rotation 363
Little’s Law 362
Mass customization 347
Modularization 343
Nesting 347
Operations process charts 359
Pareto analysis 356
Postponement 347
Process costing 351
Process flow diagram 359
Process focus 343
Process layout 350
Processes 346
Product and market focus 343
Product layout 350
Product life cycle 341
Project manufacturing 351
Repetitive manufacturing 350
Self-directed teams 363
Simplification 342
Simultaneous engineering 345
Specialization 343
Special-purpose machinery 349
Standard 342
Standardization 343
Sustainability 345
Variable costs 352
qUEsTIONs
1. Describe simplification, standardization, and specialization. Why are they important and why
are they interrelated?
2. What are the advantages and disadvantages of standardization?
3. What are the advantages and disadvantages of specialization?
4. What are product focus and process focus based on? What is a focused factory?
366
Chapter Fourteen
5. What is the advantage to modular design?
6. How are production costs affected by
a. Standard sizes?
b. Universal fit parts?
7. What are the three criteria for designing a product?
8. Why is product design important to operations costs?
9. Why is product design important to quality?
10. What is simultaneous engineering, and what are some of its advantages?
11. What is a process? What is process nesting?
12. What are the five basic factors that must be considered when designing a process?
13. Why are product design, quantity to produce, and process design intimately related?
14. Give four reasons why companies will make a product in-house and why they will outsource.
15. Describe general-purpose and special-purpose machinery. Compare each for flexibility of use,
operator involvement, run time per piece, setup time, quality, capital cost, and application.
16. What is flow processing, and what are its advantages and disadvantages?
17. What is intermittent processing and when is it used? Contrast it with flow processing.
18. When is project processing used?
19. Define fixed and variable costs and give examples of each in manufacturing. What is total cost,
and what is the equation for it?
20. What is the cost equalization point?
21. How can the variable cost be reduced? What does this do to the fixed costs, and what is needed
to economically justify this course of action?
22. What are the six steps in continuous process improvement?
23. Name and describe the two considerations in selecting a job to be studied.
24. What is a Pareto diagram, and why is it useful?
25. What is a cause-and-effect diagram, and why is it useful?
26. Why is it necessary to record?
27. Describe each of the following as it relates to recording:
a. Process boundaries.
b. Process flow.
c. Process inputs and outputs.
d. Process components.
e. Suppliers.
f. Environment.
28. What are the six symbols used in method analysis? Are there other symbols that can be used?
29. Describe each of the following:
a. Operations process chart.
b. Process flow diagram.
30. What is the purpose of the analysis step in continuous process improvement? What is the basic
question?
31. What are the four approaches that should be taken when developing a better method?
32. What are the four principles of motion economy?
33. Describe job design.
34. What is the learning curve?
35. What might be some of the advantages and disadvantages of self-directed work teams?
36. How might the learning curve impact standard production times used for planning? How would
you possibly deal with this impact?
37. How might an application of Little’s Law prove useful?
38. What is meant by the term “business environment”?
39. Select a product with which you are familiar. How do you think it might be redesigned to make
it easier to manufacture and possibly more useful to the user?
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367
PROblEms
14.1. Given the following fixed and variable costs and the volumes, calculate the total and
unit costs.
Fixed
cost
Variable
cost
Volume
(units)
$200.00
$8.00
100
$200.00
$7.00
1000
$50.00
$15.00
50
$800.00
$2.00
2000
$500.00
$20.00
500
Total
cost
Unit
cost
14.2. A process costs $200 to set up. The run time is 5 minutes per piece and the run cost
is $30 per hour. Determine the following:
a. The fixed cost.
b. The variable cost.
c. The total cost and unit cost for a lot of 600.
d. The total cost and unit cost for a lot of 1200.
Answer. a. Fixed cost = $200.00
b. Variable cost = $2.50
c. Total cost = $1450.00
d. Unit cost = $2.90
e. Total cost = $2700.00
f. Unit cost = $2.70
14.3. A manufacturer has a choice of purchasing and installing a heat-treating oven or
having the heat treating done by an outside supplier. The manufacturer has developed the following cost estimates:
Heat Treat In-house
Purchase Services
Fixed cost
$28,000.00
$0.00
Variable cost
$10.00
$17.00
a. What is the cost equalization point?
b. Should the company have the heat treating done by an outside supplier if the annual volume is 3000 units? 5000 units?
c. What would be the unit (average) cost for the selected process for each of the volumes in b above?
Answer. a. CEP = 4000 units
b. Volume 3000 units purchase services; Volume 5000 units heat treat in-house
c. Unit cost for 3000 units is $17.00; for 5000 units is $15.60
14.4. Bananas are on sale at the Cross Towne store for 79¢ per pound. They normally sell
for 99¢ per pound at your corner store. If round-trip bus fare costs $3.60 to the Cross
Towne store, is it worth going? What is the cost equalization point? Discuss other
ways of taking advantage of this bargain.
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Chapter Fourteen
14.5. Given the following costs, which process should be used for an order of 400 pieces
of a given part? What will be the unit cost for the process selected?
Process A
Process B
Setup
Buy
$40.00
$130.00
Tooling
$15.00
$20.00
Labor/unit
$4.10
$3.90
Material/unit
$2.00
$2.00
Purchase cost
$6.10
14.6. The Light Company is planning on producing a new type of light shade, the parts for
which may be made or bought. If purchased, they will cost $2 per unit. Making the parts
on a semiautomatic machine will involve a $5000 fixed cost for tooling and $1.30 per
unit variable cost. The alternative is to make the parts on an automatic machine. The
tooling costs are $15,000, but the variable cost is reduced to 60¢ per unit.
a. Calculate the cost equalization point between buying and the semiautomatic
machine.
b. Calculate the cost equalization point between the semiautomatic and automatic
machines.
c. Which method should be used for expected sales of the following?
i. 5000 units
ii. 6000 units
iii. 8000 units
iv. 10,000 units
v. 25,000 units
d. What is the unit cost for the selected process for each of the sales in c above?
Answer.
i. Unit cost = $2.00
ii. Unit cost = $2.00
iii. Unit cost = $1.925
iv. Unit cost = $1.80
v. Unit cost = $1.20
14.7. A major mail order house collected data on the reasons for return shipments over
a 3-month period with the following results: wrong selection 62,000; wrong size
50,000; order canceled 15,000; wrong address 3000; other 15,000. Construct a
Pareto diagram.
Reason
Number
Percent
Cumulative
Percent
Total
14.8. A firm experienced abnormal scrap and collected data to see which parts were causing the problem, with the following results: part A—$5720, part B—$10,500, part
Products and Processes
369
C—$890, part D—$1130, and part F—$700. Complete the following table, listing
the errors in descending order of importance. Construct a Pareto diagram.
Part
Number
Percent
Cumulative
Percent
Total
14.9. A process has a production rate of 30 units per hour. If it takes 50 minutes for a
single unit to be completed in the process, what is the inventory of units in the
process?
14.10. Draw an operations process chart for the assembly of a ballpoint pen. The pen is
made from three subassemblies (see figure):
1
2
3
4
S/A – 1
5
6
1. upper barrel
2. cartridge
3. lower barrel
Operation 1
2
3
4
5
6
Inspection 1
Operation 7
8
9
Inspection 2
3
attach clip to upper barrel
insert button into upper barrel
insert rotor into upper barrel
press ball into tip of cartridge
insert tip into cartridge
fill cartridge with ink
test cartridge
insert cartridge into upper barrel
slip spring over cartridge
screw lower barrel and upper barrel together
test operation of pen
final inspection
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Chapter Fourteen
14.11. Draw a process flow diagram for your activities from the time you wake up until
you arrive at work. Can you think of ways to improve the process?
14.12. A process can produce a final product at the rate of 2 products per minute. If an
individual product takes 30 minutes to pass through the process, what is the inventory of products in the process at any time?
14.13. A university registration process can register a person every 4 minutes. If there are
an average of 12 people in line for the registration, how long will each person have
to wait in line?
casE sTUdy 14.1
Cheryl Franklin, Production Manager
Cheryl Franklin had been production manager at Cooper Toy Company for only a week
when she started to wonder if her promotion had really been a good thing. Her boss was
concerned that production was falling behind on one of their newest toys. The company
had spent a lot to advertise the toy and the demand that had been created was not being
met by production.
Cheryl decided to try to investigate to discover the problem and try to get the production volume back on track. Unfortunately, she found that almost everyone involved was
blaming someone else.
The toy in question was one of those plastic water guns popular at beaches and swimming pools. Water is held in a reservoir and then air is pumped into the gun, allowing a
fairly large stream of water to be shot about 10 meters.
Cheryl knew that the slowest part of the production line that she was using to produce the
toy should be able to produce one every 3 minutes, and she also knew that the demand averaged only about one toy every 5 minutes. When she started her investigation at the assembly
area, however, she found that assembly was averaging only one toy every 10 minutes. The
assembly supervisor had a good explanation, however. He said that they didn’t have enough
inventory to work on. They were getting the water tanks being released from inspection on
average only one every 8 minutes, so they were assembling the toys as fast as they could.
When Cheryl went to the inspection area, she discovered that there were lots of tanks
waiting there to be inspected. The inspector assigned to the tanks appeared to be working
hard, but obviously that was the problem.
Cheryl talked to the quality manager and got quite a bit more information. To quote
the quality manager, “Look, we need to put those tanks through some important testing.
If a child pumps too much air into the tank they are designed to let some of the air out
through a relief valve. Unfortunately, a lot of the tanks have improperly glued seams.
If one of those bad tanks gets on a toy then the tank is liable to rupture before the relief
value activates. That rupture could send pieces of plastic flying. The last thing we need is
to have some child get injured with flying plastic. We find that we have to inspect every
tank since the reject rate on the gluing of the seams has been running close to 20 percent!”
Cheryl continued her investigation and discovered the following:
The supervisor of the production area where the glue was applied claims the machine
used to spread the glue will often clog, and even when it works, the glue will often be
applied unevenly.
■■ The maintenance people in charge of the glue machine claim the machine doesn’t
work well because the glue fixture is not correct for the application and the glue itself
is too thick for the type of application.
■■ The engineers in charge of the design and installation of the glue fixture claim that
the toy was rushed into production so quickly that they did not have time to design a
proper fixture so they were forced to use an old one that was originally designed for a
different application.
■■
Products and Processes
The purchasing agent responsible for buying the glue claims that they were not given
good specifications for the glue and therefore relied on the recommendation from the
supplier as to which glue to use.
■■ The marketing people claimed that the toy market is so competitive that they need to
get a toy to market quickly, especially given that the life cycle of the average toy is
very short.
Cheryl needs your help. Specifically, develop answers to the following:
■■
assignment
1. What are the underlying problems here? Try to be as specific as possible.
2. What is the best solution for these problems? Again, try to be specific.
3. Suggest how continuous process improvement techniques could be applied to the
problems.
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Chapter
fIfteen
Lean ProductIon
IntroductIon
In the past several years manufacturing has become much more competitive and the global
economy is now a forceful reality. Producers in many countries can produce goods of consistently superior quality and deliver them to international markets at a competitive price
and schedule. They have responded to changing market needs and often have detected
those needs before the consumer. Because of such competition, some countries have lost
their previous edge in the manufacture of such goods as radios, televisions, cameras, and
ships.
How have some of these companies been able to do this? It is not necessarily only
because of their culture, geography, government assistance, new equipment, or cheap
labor, but because many of them practice lean production. Lean production is a philosophy that relates to the way a manufacturing company organizes and operates its business.
It includes both the approach to organizing the business and the practice of just-in-time
production (JIT). These are not magic formulas or a set of new techniques that suddenly
makes a manufacturer more productive. Rather, they are the very skillful and coordinated
application of existing industrial and manufacturing engineering principles.
Another philosophical approach to production, developed originally by Eliyahu
M Goldratt in his book The Goal, was adding yet another perspective to those assumptions about how companies approach production. This approach is called the Theory
of Constraints (TOC), which was introduced and discussed in Chapter 6, including the
scheduling approach often called drum-buffer-rope.
This chapter will introduce and discuss the concepts of lean production and JIT. It
will also elaborate more about TOC and discuss some of the environments where each of
these concepts can best be applied.
Lean ProductIon
Lean production is a concept that has evolved from JIT concepts over the past several
years. There were many less-than-successful implementations of JIT worldwide during the 1980s, in spite of the well-documented advantages and benefits resulting from
JIT. There is strong evidence that many manufacturers, eager to take advantage of the
documented advantages, attempted to implement JIT without first understanding the fully
integrated approach and impacts of such a highly integrated system. This condition led
to many disappointing implementations or outright failures of JIT implementations during that period. Part of the misunderstanding can be attributed to confusion between the
higher level JIT “philosophy” (the fundamental changes in business practices) and the
more specific JIT production, which in its simplest form can be interpreted as delivering
material to a production process “just in time” for its need.
As is often the case when fundamental concepts are not fully understood, the JIT
concepts were viewed by many companies as invalid or inappropriate for their particular
environment. In the meantime, the development of highly integrated production systems
was rapidly evolving. Manufacturing resource planning (MRP II) was recognized to be an
effective engine to drive an integrated enterprise-wide information system that is today
372
Lean Production
373
called Enterprise Resource Planning (ERP). Purchasing and logistics activities were
similarly being integrated with fundamental internal materials management principles into
an enterprise-wide approach, today called supply chain management. Similarly, the fundamental concepts of the higher level JIT approaches evolved to an enterprise-wide perspective called lean production, and the JIT term has come to be reserved for the specific
concept of JIT production, which often implies a pull production system.
Lean production, on the other hand, implies understanding and correctly implementing the major enterprise-wide changes required to truly eliminate or significantly reduce
waste in the system. It is the system-wide philosophical approach used to integrate the system toward an ultimate goal of maximized customer service with minimal system waste.
Lean production can be defined in many ways, but the most popular is the elimination
of all waste and continuous improvement of productivity. Waste means anything other than
the minimum amount of equipment, parts, space, material, production time, and workers’
time absolutely necessary to add value to the product. This means the ideal approach is
that there should be no surplus, there should be no safety stocks, and lead times should
be minimal: “If you can’t use it now, don’t make it now.” From an organizational view,
waste refers to anything that is unnecessary and adds no value in the customer’s eyes and
for which they are unwilling to pay.
The long-term result of eliminating waste is a cost-efficient, quality-oriented, fastresponse organization that is responsive to customer needs. Such an organization has the
potential to acquire a huge competitive advantage in the marketplace. While the fundamental objective focused on waste reduction, the waste reduction approaches that were
developed also led to rapid responses to market demand through reduction of lead times,
increased levels of quality, and lower production costs.
A principal result of the lean production approach is that removal or reduction of
excessive (waste) inventory or capacity between activities in a process, regardless of the
reason that inventory or capacity existed, serves to force a tighter coupling of the activities
in the system. In other words, the organization must be managed as a system instead of a
set of relatively disjointed activities. This is an important point, in that organizations can
sometime produce failed implementations as they may focus on only certain aspects of
lean production rather than recognizing that it is really an integrated system.
Adding Value
What constitutes value to the user? It is having the right parts and quantities at the right
time and place. It is having a product or service that does what the customer wants, does
it well and consistently, and is available when the customer wants it. Value satisfies the
actual and perceived needs of the customer and does it at a price the customer can afford
and considers reasonable. Another word for this is quality. Quality is meeting and exceeding customers’ expectations.
Value starts in the marketplace when marketing must decide what the customer
wants. Design engineering must design the product so it will provide the required value to
the customer. Manufacturing engineering must first design a process to make the product
and then build the product according to certain specifications. The loop is complete when
the product is delivered to the customer, as shown in Figure 15.1. If any part of the chain
does not add value for the customer, there is waste.
MARKETPLACE
(CUSTOMER)
MARKET
RESEARCH
MANUFACTURING
Figure 15.1 Product development cycle.
PRODUCT
DESIGN
MANUFACTURING
ENGINEERING
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Chapter fifteen
Adding value to a product does not mean adding cost. Users are not concerned with
the manufacturer’s cost but only with the price they must pay and the value they receive.
Many activities increase cost without adding value and, as much as possible, these activities should be eliminated.
Waste
Anything in the product development cycle that does not add value to the product is
waste. This section will look at the causes of waste in each element of the product cycle.
Waste Caused by Poor Product Specification and Design
The creation of waste tends to start with the policies set by management in responding to
the needs of the marketplace. Management is responsible for establishing policies for the
market segments the company wishes to serve and for deciding how broad or specialized
the product line is to be. These policies affect the costs of manufacturing. For example, if
the range and variety of product are large, production runs will be short and then machines
must be changed over frequently. There will be little opportunity to use specialized
machinery and fixtures. On the other hand, a company with a limited product range can
probably produce goods on an assembly line basis and take advantage of special-purpose
machinery. In addition, the greater the diversity of products, the more complex the manufacturing process becomes, and the more difficult it is to plan and control.
Component standardization As stated in Chapter 14, companies can specialize in
the products they make and still offer customers a wide range of options. If companies
standardize on the component parts used in the different models they make, they can
supply customers with a variety of models and options made from standard components.
Parts standardization has many advantages in manufacturing. It creates larger quantities of specific components that allow longer production runs. This, in turn, makes it
more economical to use more specialized machinery, fixtures, and assembly methods.
Standardization reduces the planning and control effort needed, the number of items
required, and the inventory that has to be carried.
The “ideal” product is one that meets or exceeds customer expectations, makes the
best use of material, and can be manufactured with a minimum of waste (at least cost). As
well as satisfying the customer, the product’s design determines both the basic manufacturing processes that have to be used and the cost and quality of the product. The product
should be designed so it can be made by the most productive process with the smallest
number of operations, motions, and parts and includes all of the features that are important
to the customer. Chapter 14 discussed the principles of product design in more detail.
Waste Caused in Manufacturing
Manufacturing takes the design and specifications of the product and, using the manufacturing resources, converts them into useful products. First, however, manufacturing engineering
must design a process capable of making the product. They do so by selecting the manufacturing steps, machinery, and equipment and by designing the plant layout and work methods.
Manufacturing must then plan and control the operation to produce the goods. This involves
manufacturing planning and control, quality management, maintenance, and labor relations.
Shigeo Shingo identified seven important sources of waste in manufacturing. The
first four relate to the design of the manufacturing system and the last three to the operation and management of the system:
1. Processing. The best process is one that has the capability to consistently make the
product with an absolute minimum of scrap, in the quantities needed, and with the
least cost added. Waste, or cost, is added to the process if the wrong type or size of
machine is used, if the process is not being operated correctly, or if the wrong tools
and fixtures are used. Overprocessing is a redundant effort that adds no value to the
product or service.
Lean Production
375
2. Motion. Waste is added if the methods of performing tasks by the operators cause
wasted movement, time, or effort. Activities that do not add value to the product
should be eliminated. Searching for tools, walking, or unnecessary motions are all
examples of waste of motion.
3. Transportation. Moving and storing components adds cost but not value. For example, goods received may be stored and then issued to production. This requires labor to
put away, find, and deliver to production. Records must be kept and a storage system
maintained. Poorly planned layouts may make it necessary to move products over long
distances, thus increasing the movement cost and possibly storage and recordkeeping
costs. Any movement that does not directly support immediate use is considered waste.
4. Defects. Defects interrupt the smooth flow of work. If the scrap is not identified, the
next workstation receiving it will waste time trying to use the defective parts or waiting for good material. Schedules must be adjusted. If the next step is the customer,
then the cost will be even higher. Sorting out defects or the repair or reworking of
products or services are also waste.
5. Waiting time. There are two kinds of waiting time: that of the operator and that of
material. If the operator has no productive work to do or there are delays in getting
material or instructions, there will be waste. Ideally, material passes from one work
center to the next and is processed without waiting in queue. Waste occurs when
activities are not fully synchronized.
6. Overproduction. Overproduction is producing products beyond those needed for
immediate use. When this occurs, raw materials and labor are consumed for parts not
needed, resulting in unnecessary inventories. Considering the costs of carrying inventory, this can be very expensive. Overproduction causes extra handling of material,
extra planning and control effort, and quality problems. Because of the extra inventory and work-in-process, overproduction adds confusion, tends to bury problems
in inventory, and often leads to producing components that are not needed instead
of those that are. Overproduction is not necessary as long as market demand is met.
Machines and operators do not always need to be fully utilized.
7. Inventory. As discussed in Chapter 9, inventory costs money to carry, and excess
inventory adds extra cost to the product. However, there are other costs in carrying
excess inventory. Any supply of inventory, from raw materials to finished goods, in
excess of immediate requirements is waste.
To remain competitive, a manufacturing organization must produce better products at
lower cost while responding quickly to the marketplace. The role inventory plays in each
of these steps is discussed below.
A better product suggests one that has features and quality superior to others. The
ability to take advantage of product improvement opportunities depends on the speed
with which engineering changes and improvements can be implemented. If there are large
quantities of inventory to work through the system, it takes longer and is more costly prior
to engineering changes reaching the marketplace.
Lower inventories also improve quality. Suppose that a component is made in batches
of 1000 and that a defect occurs on the first operation. Eventually, the defect will be
caught, very often after several more operations have been completed. Thus, all 1000
pieces have to be inspected. Because much time has elapsed since the first operation when
the defect occurred, it is also difficult to pinpoint the cause of the problem. If the batch
size had been 100 instead of 1000, it would have moved through the system more quickly
and been detected earlier, and there would only be 100 to inspect.
Companies can offer better prices if their costs are low. Lower inventories reduce
cost. Also, if work-in-process inventory is reduced, less space is needed in manufacturing,
resulting in cost savings.
Responsiveness to the marketplace depends on being able to provide shorter lead times
and better due date performance. In Chapter 6 it was observed that manufacturing lead time
depends on queue, and queue depends on the number and the batch size of the orders in
process. If batch size is reduced, the queue and lead time will be reduced. Chapter 8 noted
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Chapter fifteen
that forecasts were more accurate for nearer periods of time. Reducing lead time improves
forecast accuracy and provides better order promising and due-date performance.
Poka-Yoke (Fail Safe)
Poka-yoke was introduced by Shigeo Shingo of Japan. It implies the concept of
removing faults at the first instance and making a process or product “foolproof.”
Shingo argued that statistical quality control does not prevent defects. He differentiated between errors and defects, stating that errors will always be made, but defects
can be prevented. Errors are primarily related to the process while defects primarily
are associated with the product. Process errors are not always directly related to the
creation of a defect, but usually imply the need for focused inspection at least until
process improvement can be made. Corrective action should take place immediately
after a mistake is made, which implies 100% inspection as soon as an action occurs.
This inspection can take one of three forms: successive check, self-check, and source
inspection. Successive check inspection is done by the next person in the process who
passes the information back to the worker who just performed the operation who can
then make any needed repair. Self-check is done by the worker and can be used on all
items where a sensory perception is sufficient. Scratches and paint blobs are examples
of these. Source inspection also is done by the individual worker who, instead of
checking for defects, checks for errors that will cause defects.
Poka-yoke tries to change either the process or its resources, thus eliminating the need
to rely on human experience and knowledge. Examples include the following:
Use color-coded parts.
Put a template over an assembled component to show operators where specific parts go.
■■ Use counters to tell an operator how many operations have been performed.
■■ Have one prong larger than the other so the electric plug will fit only one way, such as
found in electrical wiring.
■■
■■
the Lean ProductIon envIronment
Many elements are characteristic of a lean production environment. They may not all exist
in a particular manufacturing situation, but in general they provide some principles to help
in the development of a lean production system. These can be grouped under the following headings:
Flow manufacturing.
Process flexibility.
■■ Quality management.
■■ Total productive maintenance.
■■ Uninterrupted flow.
■■ Continuous process improvement.
■■ Supplier partnerships.
■■ Total employee involvement.
■■
■■
Flow Manufacturing
The lean production concept was developed by companies such as Toyota and some major
appliance and consumer electronics manufacturers. These companies manufacture goods
in a repetitive manufacturing environment.
Repetitive manufacturing is the production of discrete units on a flow basis. In
this type of system, the workstations required to make the product, or family of products, are located close together and in the sequence needed to make the product. Work
flows from one workstation to the next at a relatively constant rate and often with some
material handling system to move the product. Figure 15.2 shows a schematic of flow
manufacturing.
Lean Production
377
WORKSTATIONS
Input
1
2
3
4
Output
Figure 15.2 Flow manufacturing.
Saw
Grinder
Saw
Saw
Lathe
Lathe
Lathe
Grinder
Drill
Drill
Drill
Figure 15.3 Functional layout.
Saw
Lathe
Grinder
Drill
Lathe
Figure 15.4 Work cell layout.
These systems are discussed in Chapter 14. They are suitable for a limited range of
similar products such as automobiles, televisions, or microwave ovens. Because work centers are arranged in the sequence needed to make the product, the system is not suitable for
making a variety of different products. Therefore, the demand for the family of products
must be large enough to justify economically setting up the line. Flow systems are usually
very cost effective.
Work cells Many companies do not have a product line that lends itself to flow manufacturing. For example, many companies do not have sufficient volume of specific products to justify setting up a line. Companies with this kind of product line usually organize
their production on a functional basis by grouping together similar or identical operations.
Lathes will be placed together, as will milling machines, drills, and welding equipment.
Figure 15.3 shows a schematic of this kind of layout, including routing for a hypothetical
product (saw, lathe, grinder, lathe, drill). Product moves from one workstation to the other
in lots or batches. This type of production produces long queues, high work-in-process
inventory, long lead times, and considerable material handling.
Usually this kind of layout can be improved. It depends on the ability to detect product
flows. This can be done by grouping products together into product families. Products are
in the same family if they use common work flow or routing, materials, tooling, setup procedures, and cycle times. Workstations can then be set up in miniature flow lines or work
cells. For example, suppose the product flow shown in Figure 15.3 represents the flow for a
family of products. The work centers required to make this family can be laid out according
to the steps to make that family. Figure 15.4 shows a schematic of such a layout.
Parts can now pass one by one, or in very small lots, from one workstation to the next.
This has several benefits:
Queue and lead times going through the cell are reduced drastically.
Production activity control and scheduling are simplified. The cell has only one work
center to control as opposed to five in a conventional system.
■■ Floor space needed is reduced.
■■ Feedback to preceding operations is immediate. If there is a quality problem, it will be
found out immediately.
■■
■■
Work cells permit high-variety, low-volume manufacturing to be repetitive. For work
cells to be really effective, product design and process design must work together so parts
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Chapter fifteen
Machine
A
W
I
P
Machine
B
Large
Machine
C
W
I
P
Machine
D
W
I
P
Machine
E
Flow with Large-Capacity Nondedicated Equipment
Machine A
Small C1
Machine D
Machine B
Small C2
Machine E
Flow with Small-Capacity Dedicated Equipment
Figure 15.5 Large versatile equipment versus small dedicated equipment.
are designed for manufacture in work cells. Component standardization becomes even
more important. Work cells are sometimes called cellular manufacturing.
Process Flexibility
Process flexibility is desirable so the company can react swiftly to changes in the volume
and mix of their products. To achieve this, operators and machinery must be flexible, and
the process must be configured to be changed over quickly from one product to another.
Machine flexibility To achieve machine flexibility, it often makes more sense to have
two relatively inexpensive general-purpose machines than one large, expensive specialpurpose piece of equipment. Smaller general-purpose machines can be adapted to particular jobs with appropriate tooling. Having two instead of one makes it easier to dedicate
one to a work cell. Ideally, the machinery should be low-cost and movable. Figure 15.5
illustrates the concept.
Quick changeover Quick changeover requires short setup times, and improves
responsiveness to customer changes. Short setup times also have the following advantages:
Reduced order quantity. The lot size is often dependent on the setup cost. If the setup
time can be reduced, the lot size can be reduced. For example, if the order quantity
is 100 units and the setup can be reduced to 25% of its former value, the order quantity decreases to 50. Inventory is cut in half, and queue and lead times are reduced.
Reductions in setup of even greater magnitude are possible. The general opinion is that
setup can be cut 50% simply by organizing the work and having the right tools and fixtures available when needed. For example, in one instance, a changeover on a die press
was videotaped. The operator doing the changeover was not in view for more than 50%
of the time, as he was away from the machine getting tools, dies, and so on. One system
for setup reduction, called the “four-step method,” claims that reductions of 90% can be
achieved without major capital expense. This is accomplished by organizing the preparation, streamlining the setup, and eliminating adjustments.
■■ Reduced queue and manufacturing lead time. Manufacturing lead time depends
mostly on the queue. In turn, queue depends on the order quantity and scheduling.
Reducing setup time reduces the order quantity and queue and lead times.
■■ Reduced work-in-process (WIP) inventory. Work-in-process inventory depends
on the number of orders in process and their size. If the order quantity is reduced, the
WIP is reduced. This frees up more floor space, allowing work centers to be moved
closer together, thus reducing handling costs and promoting the creation of work cells.
■■ Improved quality. When order quantities are small, defects have less time to be buried. Because they are more quickly and easily exposed, their cause will more likely be
detected and corrected.
■■
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■■
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Improved process and material flow. Inventory acts as a buffer, burying problems
in processes and in scheduling. Reducing inventory reduces this buffer and exposes
problems in the production process and in the materials control system. This gives an
opportunity to correct the problems and improve the process.
A lean principle that applies to quick changeover is single minute exchange of die,
or SMED. SMED is a methodology that identifies which setup activities can be done
while the machine is running (external setup) and which can only be done while the
machine is not operating (internal setup). Once defined, a process is put in place to
convert internal steps to external steps when possible, organize all tools required to support the activities, streamline all steps, and constantly evaluate the process for opportunities to improve.
Operator flexibility Flexible machinery and flexible processes need flexible people to operate them. To achieve operator flexibility, people should not only be trained
in their own jobs but should also be cross-trained in other skills and in problem-solving
techniques. Only with well-trained people can the benefits of process flexibility be
realized.
Standardized work Standardized work is the process of documenting and standardizing
tasks through the value stream, including processes, best practices, and standards. These then
become the baseline for continuous improvement activities. As the standard is improved, it
becomes the new baseline. Fundamentals for standardized work include utilizing best practices,
and determining the who, what, where, when, why, and how of a process. Standardization is
not the goal, but is the foundation used to drive continuous improvement, and can be applied at
all levels and in all areas of a business.
Quality Management
Quality management is the subject of Chapter 16. This section will focus on quality from
the point of view of manufacturing and lean production.
Quality is important for two reasons. If quality is not present in what is supplied to
the customer and the product is defective, the customer will be dissatisfied. If a process
produces scrap, it creates disrupted schedules that delay supplying the customer, increases
inventory or causes shortages, wastes time and effort on work centers, and increases the
cost of the product.
Who is the user? Ultimately, it is the company’s customer, but the user is also the
next operation in the process. Quality at any one work center should meet or exceed the
expectations (needs) of the next step in the process. This is important in maintaining the
uninterrupted flow of material. If defects occur at one work center and are not detected
until subsequent operations, time will be wasted, and the quantity needed will not be
supplied.
For manufacturing, quality does not mean inspecting the product to segregate good
from bad parts. Manufacturing must ensure that the process is capable of producing the
required quality consistently and with as close to zero defects as possible. Manufacturing
must do all it can to improve the process to achieve this and then monitor the process to
make sure it remains in control. Daily monitoring can best be done by the operator. If defects
are discovered, the process should be stopped, and the cause of the defects corrected.
The benefits of a good-quality program are less scrap, less rework, less inventory
(inventory just in case there is a problem), better on-time production, timely deliveries,
and more satisfied customers.
As the concepts of quality management evolved, several of the concepts were structured, formalized, and enhanced to eventually become six sigma quality. Several of the
key aspects of the quality approach are discussed in Chapter 16.
Quality at the source Quality at the source means doing it right the first time and,
if something does go wrong, stopping the process and fixing it. People become their own
inspectors, personally responsible for the quality of what they produce.
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Total Productive Maintenance
Traditional maintenance might be called breakdown maintenance, meaning maintenance is done only when a machine breaks down. The motto of breakdown maintenance
is “If it ain’t broke, don’t fix it.” Unfortunately, breakdowns occur only when a machine
is in operation, resulting in disrupted schedules, excess inventory, and delayed deliveries. In addition, lack of proper maintenance results in wear and poor performance. For
example, if a car is not properly maintained, it will break down, not start, or perform
poorly on the road.
For a process to continue to produce the required quality, machinery must be maintained in excellent condition. This can best be achieved through a program of preventive
maintenance. This is important for more reasons than quality. Low work-in-process
inventories mean there is little buffer available. If a machine breaks down, it will quickly
affect other work centers. Preventive maintenance starts with daily inspections, lubrication, and cleanup. Since operators usually understand how their equipment should “feel”
better than anyone else, it makes more sense to have them handle this type of regular
maintenance.
Total productive maintenance takes the ideas of preventive maintenance one step
further. According to APICS Dictionary, 14th edition, total productive maintenance is
“preventive maintenance plus continuing efforts to adapt, modify, and refine equipment
to increase flexibility, reduce material handling, and promote continuous flow.” As such,
it emphasizes the lean production principle of eliminating waste. Machine operators are
typically a major part of a total productive maintenance program. Not only can they be
involved with small daily maintenance responsibilities and lower-skilled routine tasks, but
they can also assist skilled repair technicians who work on the equipment should a breakdown occur. They can also help the skilled technicians on start-up and shutdown tasks.
An additional advantage of operator involvement is the reduction in time and work
requirements on the skilled technicians, allowing those skilled technicians more time to
focus on equipment overhaul and processing improvements.
Uninterrupted Flow
Ideally, material should flow smoothly from one operation to the next with no delays.
This is most likely to occur in repetitive manufacturing, where the product line is limited
in variety. However, the concept should be the goal in any manufacturing environment.
Several conditions are needed to achieve uninterrupted flow of materials: uniform plant
loading, a pull system, valid schedules, and linearity.
Uniform plant loading Uniform plant loading means that the work done at each
workstation is distributed to be as close to equal as possible. In repetitive manufacturing,
this is called line balancing, which means that the time taken to perform tasks at each
workstation on the line is the same or very nearly so. The result will be no bottlenecks and
no buildup of work-in-process inventory.
Valid schedules There should be a well-planned valid schedule. The schedule sets
the flow of materials coming into the factory and the flow of work through manufacturing. To maintain an even flow, the schedule must be level. In other words, the same
amount should be produced each day. Rate-based scheduling can be used, which
bases the production on a rate for a given period, whether it be hour, day, or week.
Furthermore, the mix of products should be the same each day. For example, suppose a
company makes a line of dog clippers composed of three models: economy, standard,
and deluxe. The demand for each is 500, 600, and 400 per week, respectively, and the
capacity of the assembly line is 1500 per week. The company can develop the schedule
shown in Figure 15.6. This will satisfy demand and will be level based on capacity.
However, inventory will build up and, if there is no safety stock, a variation in demand
will create a shortage. For example, if there is a surge in demand for the deluxe model in
week 1, none may be available for sale in week 2.
Lean Production
Week
On
Hand
Economy
0
Standard
600
Deluxe
800
Total
1400
1
2
3
1500
300
381
1500
1200
1500
1500
1500
Figure 15.6 Master production schedule.
Week
On
Hand
1
2
3
Economy
250
500
500
500
Standard
300
600
600
600
Deluxe
200
400
400
400
Total
750
1500
1500
1500
Figure 15.7 MPS leveled by week.
An alternate schedule is shown in Figure 15.7. With this schedule, inventory is
reduced, and the ability to respond to changes in model demand increases. The number of
setups increases, but this is not a problem if setup times are small. The idea can be carried
further by making some of each model each day. Now it would mean producing 100, 120,
and 80 of each model each day. If the line has complete flexibility, these can be produced
in the following mixed sequence of 15. This is repeated 20 times during the day for a total
output of 300:
Sequence: ESD, ESD, ESD, ESD, SES
E: Economy
S: Standard
D: Deluxe
The company makes some of everything each day in the proportions to meet demand.
Inventories are at a minimum. If demand shifts between models, the assembly line can
respond daily. This is called mixed-model scheduling. The schedule is leveled, not only
for capacity, but also for material.
No matter what type of scheduling is used in a lean environment, the critical principle
is that it be leveled, or smoothed, as much as possible, to facilitate the use of kanbans, and
to eliminate waste. Another term for this is heijunka, which is a strategy for redistributing production volume and mix in order to minimize extremes and meet varying customer
demand. APICS Dictionary, 14th edition, defines it as “an approach to level production
throughout the supply chain to match the planned rate of end product sales.”
Linearity The emphasis in lean production is on achieving the plan—no more, no less.
This concept is called linearity and is usually reached by scheduling to less than full capacity. If an assembly line can produce 100 units per hour, it can be scheduled for perhaps
700 units for an eight-hour shift. If there are problems during the shift, there is extra time
so the 700-unit schedule can be maintained. If there is time left over after the 700 units are
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produced, it can be spent on jobs such as cleanup, lubricating machinery, getting ready
for the next shift, quality improvements, total productive maintenance activities, process
improvements, or solving problems.
Continuous Process Improvement
This topic is an element in both lean production and quality management and is discussed
in Chapters 14 and Chapter 16. Elimination of waste depends on improving processes
continuously. Thus, continuous improvement is a major feature of lean production.
Supplier Partnerships
If good schedules are to be maintained and the company is to develop a lean environment, it
is vital to have good, reliable suppliers. They establish the flow of materials into the factory.
Partnering Partnering implies a long-term commitment between two or more organizations to achieve specific goals. Lean production places much emphasis not only on supplier
performance but also on supplier relations. Suppliers are looked on as coproducers, not as
adversaries. The relationship with them should be one of mutual trust and cooperation.
There are three key factors in partnering:
1. Long-term commitment. This is necessary to achieve the benefits of partnering. It
takes time to solve problems, improve processes, and build the relationship.
2. Trust. Trust is needed to eliminate an adversarial relationship. Both partners must
be willing to share information and form a strong working relationship. Open and
frequent communications are necessary. In many cases the parties have access to each
other’s production plans and technical information.
3. Shared vision. All partners must understand the need to satisfy the customer. Goals
and objectives should be shared so that there is a common direction.
If properly done, partnering is a win–win situation. The benefits to the buyer include
the following:
The ability to supply the quality needed 100% of the time so there will be no need for
inbound inspection. This implies that the supplier will have, or develop, an excellent
process quality improvement program.
■■ The ability to make frequent deliveries on a just-in-time basis, which implies that the
supplier will apply lean principles to become a supply chain partner.
■■ The ability to work with the supplier to improve performance, quality, and cost. For a
supplier to become a valuable supply chain supplier, a long-term relationship must be
established. Suppliers need to have that assurance so they can plan their capacity and
make the necessary commitment to a single customer.
■■
In return, the supplier has the following benefits:
A greater share of the business with long-term security.
■■ Ability to plan more effectively.
■■ A competitive advantage as a lean supplier.
■■
Supplier selection Chapter 7 noted that the factors to be considered when selecting
suppliers were technical ability, manufacturing capability, reliability, after-sales service,
supplier location, and lean capabilities. In a partnership there are other considerations
based on the partnership relationship. They include the following:
1. The supplier has a stable management system and is sincere in implementing the partnership agreement.
2. There is no danger of the supplier breaching the organization’s proprietary information.
3. The supplier has an effective quality system.
4. The supplier shares the vision of customer satisfaction and delighting the customer.
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Supplier certification Once the supplier is selected, the next step is a certification process that begins after the supplier has started to ship the product. Organizations can set up
their own criteria for supplier certification or can use one such as what has been developed
by the American Society for Quality, whose criterion emphasizes the absence of defects
both in product and nonproduct categories (e.g., billing errors) and use of a good documentation system, such as the ISO 9000:2015 system, which is discussed in Chapter 16.
Total Employee Involvement
A successful lean production environment can be achieved only with the cooperation
and involvement of everyone in the organization. The ideas of elimination of waste and
continuous improvement that are central to the lean production philosophy can be accomplished only through people cooperating.
Instead of being receivers of orders, operators must take responsibility for improving
processes, controlling equipment, correcting deviations, and becoming vehicles for continuous
improvement. Their jobs include not only direct labor but also a variety of traditionally indirect
jobs such as preventive maintenance, some setup, data recording, and problem solving. As
discussed previously in this chapter, employees must be flexible in the tasks they do. Just as
machines must be flexible and capable of quick changeover, so must the people who run them.
The role of management must change. Traditionally, management has been responsible for planning, organizing, and supervising operations. In a lean environment many of
their traditional duties are done by line workers. More emphasis is placed on the leadership role, meaning managers and supervisors must become coaches and trainers, develop
the capability of employees, and provide coordination and leadership for improvements.
Traditionally, staff have been responsible for such things as quality control, maintenance, and recordkeeping. Under lean production, line workers do many of these duties.
Staff responsibilities then become those of training and assisting line workers to do the staff
duties assigned to them.
manufacturIng PLannIng and controL In a Lean
ProductIon envIronment
The philosophy and techniques of lean production discussed in this chapter are related
to how processes and methods of manufacture are designed. The major responsibility
for designing processes and methods lies with manufacturing and industrial engineering.
Manufacturing planning and control are responsible for managing the flow of material
and work through the manufacturing process, not designing the process. However, manufacturing planning and control are governed by, and must work with, the manufacturing
environment, whatever it is. Figure 15.8 shows the relationship.
No matter what planning and control system is used, these four basic questions have
to be answered:
1. What are we going to make?
2. What do we need to make it?
3. What do we have?
4. What must we get?
The logic of these questions always applies, whether cooking a meal or making a jet
aircraft. Systems for planning and control vary based on the industry and product. The manufacturing planning and control system discussed in this text has proved effective in any manufacturing environment. The complexity of the manufacturing process, the number of finished
items and parts, the levels in the bill of material, and the lead times have made the planning
and control problems either simple or complex. If anything can be done to simplify these factors, the planning and control system will be simpler. In general, lean production simplifies
these factors, thus making the planning and control problems easier to identify and resolve.
The sections that follow will look at how the various parts of the manufacturing
planning and control system relate to a lean production environment. In general, lean
production does not make the manufacturing planning and control system obsolete but, in
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LEAN PRODUCTION
MANUFACTURING
PLANNING AND CONTROL
PROCESS DESIGN
Forecasting
Flow Manufacturing
Master Planning
Process Flexibility
Material Requirements Planning
Total Quality Management
Capacity Management
Uninterrupted Flow
Production Activity Control
Total Employee Involvement
Purchasing
Supplier Partnerships
Figure 15.8 Lean production.
some ways, does change the focus. Lean production is not primarily a planning and control system. It is a philosophy and a set of techniques for designing and operating a manufacturing plant. Planning and control are still needed in a lean production environment.
Forecasting
The major effect that lean production has on forecasting is shortened lead time. This does
not affect forecasting for business planning or production planning, but it does for master
production scheduling. If lead times are short enough that production rates can be matched
to sales rates, forecasting for the master production schedule becomes less important.
Sales and Operations Planning/Production Planning
Long-term planning must extend far enough into the future to allow for any changes in the
production flow and required resources, long lead time purchases, and the development
of any new long-term relationships with suppliers. Lean production has the potential for
reducing long lead times, but more importantly, it provides an environment in which the
supplier and buyer can work together to plan the flow of material.
Master Production Scheduling
Several scheduling factors are influenced by lean production:
1. Master scheduling tries to level capacity, while lean production tries to level the
schedule based on capacity and material flow. Figure 15.9 illustrates the difference.
2. The shorter lead times reduce time fences and make the master production schedule
more responsive to customer demand. The ideal lead time is so short that the company can respond to actual sales, not to forecast. Whether the company builds to a
seasonal demand or to satisfy promotion, a forecast is still necessary. Planning horizons can also be reduced.
3. Lean production requires a stable schedule to operate. This principle is supported by
using time fences. These are established based on lead times and the commitment of
materials and resources. If lead times can be reduced through lean production, the
time fences can be reduced.
4. Traditionally, weekly time buckets have been used for planning. Because of reduced
lead times and schedule stability, it is possible to use daily time buckets in a lean
environment, due to the uninterrupted flow.
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Level Capacity Schedule
Week
1
Model A
900
Model B
300
Model C
Total
1200
2
3
900
300
1200
1200
1200
Level Material and Capacity Schedule
Week
1
2
3
Model A
300
300
300
Model B
400
400
400
Model C
500
500
500
Total
1200
1200
1200
Figure 15.9 MPS leveled by capacity and by material.
Material Requirements Planning
Material requirements planning (MRP) plans the material flow based on the bill of material, lead times, and available inventory. Lean practices may modify this approach in
several ways:
The MRP time buckets were originally one week. As lead times are reduced and the
flow of material improved, these can be reduced to daily buckets.
■■ The MRP netting logic is based on generating order quantities calculated using the
planned order releases of the parent, the inventory on hand, and any order quantity
logic used. In a pure lean production environment, there is very little or no inventory
on hand, and the order quantity logic is to make exactly what is needed, or lot-for-lot.
If the lead times are short enough, component production occurs in the same time
bucket as the gross requirement, and no offsetting is required.
■■ Bills of material can frequently be flattened in a lean environment. With the use of
work cells and the elimination of many inventory transactions, some levels in bills of
material become unnecessary.
■■
Both MRP and lean production are based on establishing a material flow. In a
repetitive manufacturing environment, this is set by the model mix and the flow rate. The
product to be made is decided by the need of the following workstation, which is ultimately the assembly line.
However, many production situations do not lend themselves to level scheduling and
the pull system and therefore must rely more heavily on MRP logic. Some examples are
as follows:
The demand pattern is unstable.
Custom engineering is required.
■■ Quality is unpredictable.
■■ Volumes are low and occur infrequently.
■■
■■
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Capacity Management
Capacity planning’s function is to determine the need for labor and equipment to meet
the priority plans. Leveling schedules should make the job easier, as will the lean production emphasis on cutting out waste and problems that cause ineffective use of capacity.
Capacity control focuses on adjusting capacity daily to meet demand. Capacity across
work centers is synchronized so that work flows smoothly. Leveling should make this task
easier, but so will the lean production emphasis on cutting out waste and problems that
cause ineffective use of capacity. Linearity, the practice of scheduling extra capacity, will
improve the ability to meet priority schedules.
Capacity management in some ways becomes more important in a lean production
environment. Since a lean production environment tends to have very low inventory, it
becomes critical to have the right capacity at the right time to make the inventory according to customer demand. This also relies on the importance of total productive maintenance, since that approach provides assurance that the process capacity will likely be
ready to use when needed.
Inventory Management
Because lean production reduces the inventory in the system, in some respects this should
make inventory management easier. However, if order quantities are reduced and annual
demand remains the same, more work orders and more paperwork must be tracked, and
more transactions recorded. The challenge then is to reduce the number of transactions
that have to be recorded. One system used is called backflushing or post-deduct inventory transaction processing.
Material flows from raw material to work-in-process to finished goods. In a postdeduct system, raw materials are recorded into work-in-process. When work is completed
and the raw material becomes finished goods, the work-in-process inventory is relieved by
multiplying the number of units completed by the number of parts in the bill of material.
In some cases, primarily for those products with short throughput times, material is not
electronically issued into work-in-process, meaning it is removed directly from raw material once work is completed. The system works if the bills of material are accurate and
if the manufacturing lead times are short. Backflushing can also be done at intermediate
work stations rather than waiting until finished goods for all components, to gain greater
visibility of work-in-process.
One benefit of backflushing is the reduction in the number of transactions, which
reduces waste, a major target of lean production. This replacement of detailed work-inprocess accounting requires a very high level of bill of material data accuracy. A possible disadvantage with backflushing as the only accounting approach for materials is the
inability to capture possible problems with a component, such as scrapping the component
for quality problems, damage to the component, or just losing it in the inventory system.
When such problems may occur, those losses should be reported to the transaction system
to prevent possible future shortages of components.
Little’s Law, introduced in Chapter 14, demonstrates the impact of reducing inventory in a lean environment. By removing the need for buffer inventory through quality improvements, reduced setup times and cycle inventory, total inventory is reduced.
Assuming a relatively constant value-added production rate, the reduction in inventory
would also reduce throughput time. That reduction in time would imply the ability to
respond quicker to demand, meaning better information about the demand due to a reduction in forecast error. The improved information continues to allow for more inventory
reduction, and the continuous improvement cycle continues.
The Push System
Traditional manufacturing and inventory control uses a push system, meaning production
is performed based on an advance schedule with dates and quantities. In a distribution system, a push system replenishes warehouses based on decisions made at the central facility.
MRP is often called a push system, meaning that the material needs are calculated ahead
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387
of time (planned order releases) and, assuming there are no significant changes to the
plans, pushed out to the production system as a production order. The trigger for the entire
plan is the projection of the final product need, as represented by the master production
schedule (MPS). Part of the difficulty with MRP is that often the plans are not effective
because of problems or changes, including:
Changes in customer requirements, both in timing and quantity.
Supplier delivery problems, including timing, quantity, and quality.
■■ Inaccurate databases that can make the plans invalid.
■■ Production problems, including the following:
■■ Absenteeism in the workforce.
■■ Productivity and/or efficiency problems.
■■ Machine downtime.
■■ Quality problems.
■■ Poor communication.
■■
■■
These problems generally promote an environment that, despite the best-laid plans,
can allow for ineffective execution and a growth in inventory levels.
The Pull System
The pull system is an alternative to the push system, and is often identified as the primary
aspect of lean production. Instead of making product based on a predetermined schedule,
the pull concept produces only what is needed, when it is needed, to satisfy customer order
demand. The same concept is used throughout the production facility, each work center not
producing product until the subsequent work center has a demand. In distribution, replenishments would not be made until the warehouse had a demand for the item. Essentially, this
system is much the same as the basic reorder point system used for independent inventory.
Reorder points normally do not work well in a dependent inventory environment due
to a significant violation of the assumption of relatively constant demand that allows a
reorder point to work well in some independent inventory environments. A simple example may help illustrate the problem.
Suppose the product is a specific model of bicycle. The bicycles are made in batches,
which is a typical mode of production for an assemble-to-order environment. The batch
size is 200 bicycles.
A dependent inventory item that is one level lower on the bill of materials is the bicycle seat. Suppose it has a lot size of 300, a two-week lead time, and a reorder point of 80.
Example 1—In this case suppose there is an inventory of 290 seats. A new batch of
bicycles has just been ordered, requiring the use of 200 of the seats in a very short time.
The balance left is 90 seats, 10 above the reorder point. The seat is not reordered since the
reorder point has not been reached. The 90 will stay in inventory until the next order for
the bicycles is generated, which may be a significant time. When that order does come
to build another 200 bicycles, only 90 can be built because that is the amount of seats
in inventory. Another lot of 300 must be immediately ordered, but it will be two weeks
before they are available.
Example 2—Now assume that there are 270 seats in inventory. The order for 200
bicycles comes in, 200 seats are used, and the seat reorder point is reached, causing an
immediate reorder. Two weeks later the 300 seats arrive and are added to the 70 left in
stock. There are now 370 seats that will stay in inventory (costing a lot of money) until the
next time the bicycles are made, which may be a very long time.
As the example illustrates, the lot-sizing problem with dependent inventory often results
in either a crisis shortage or a replenishment of stock well before it is actually needed. This
example shows that the critical conditions causing the problem are the large lot sizes and the
long lead times, both of which are major targets of lean production waste reduction.
First, look again at the standard EOQ model that helps determine the most economical lot size. It is, of course, the basic trade-off of inventory holding cost and order cost, as
described in detail in Chapter 10 and shown in Figure 15.10.
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$
TOTAL COST
HOLDING COST
ORDER COST
0
EOQ
QUANTITY
Figure 15.10 Balancing total cost.
A fundamental assumption of this model is that the two major costs involved are
known and relatively fixed. While this is essentially true with holding cost, it is not true
with order cost. If the order cost is equipment setup, then a major lean production effort is
to reduce this setup cost. If it is a purchased item, the major effort is to work with suppliers
to reduce the cost and time of purchase order and delivery. With these efforts, the order
cost curve is driven downward and to the left, as shown in Figure 15.11.
$
TOTAL COST
HOLDING COST
ORDER COST
0
EOQ
QUANTITY
Figure 15.11 Downward order cost.
When these actions are taken, a new total cost curve based on the new order cost
curve is generated, resulting in a significantly smaller EOQ, as Figure 15.12 illustrates.
$
OLD TOTAL COST
HOLDING COST
NEW TOTAL COST
ORDER COST
0
NEW EOQ
OLD EOQ
Figure 15.12 New order cost curve.
QUANTITY
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389
This implies that the economic order quantities and the reorder points are very small,
meaning they will be ordered frequently but in very small batches. Since the actions
are also taken at the final product level, there will be, in the preceding bicycle example,
frequent lots of a very small quantity of bicycles built requiring small lots of seats frequently reordered.
Reviewing the scenario illustrates the impact. The lot size for the bicycles is very
small, as is the lot size for the seats. Lead time to replenish the seats, also a target for lean
production improvement, has shrunken as well. Suppose the bicycle lot size is now 7 and
the seat lot size is 10. The reorder point for the seats is now zero. If one lot size of bicycles
is built, the seat reorder point will not be reached, and there will not be enough seats to
make another lot of bicycles. With such a small seat lot size, however, it is affordable
to keep two, three, or even more lots on hand (the number being dependent on the new
replenishment lead time). Therefore, the next lot of bicycles can be produced with the second lot of seats while the first lot of seats is being replenished.
A lean production approach that emphasizes this reduction in batch size is one piece
flow. This philosophy builds on the principle that the optimal batch size is one at a time. This
reduction in batch size is reliant on eliminating the constraints that require larger batch sizes.
The downside of the change While the average inventory is clearly lower in the small
lot size scenario, there is a cost involved beyond the one-time cost to reduce the order cost
and the lead time. Given that the overall customer demand has not diminished, batches will
need to be built much more frequently since each batch is smaller in size. Each time the
inventory of a given batch gets close to the reorder point, there is a risk of stockout if the
demand during the replenishment lead time exceeds expectations. In order to maintain service levels, such as discussed in Chapter 11, a small safety stock may be required to protect
against the additional risks of stockouts. Figure 15.13 illustrates the condition.
The Kanban System
With shortened lead times a constant goal in lean production, something is needed to generate the reorder point signal without having to rely on a formal, structured system that
could take time to react. The developers of the JIT production concepts utilized a simple
card system called kanban, which roughly translated from Japanese means card or sign.
The system works very simply. The kanban signal identifies the material to which it is
attached. The information on the kanban will often include the following:
Component part number and identification.
Storage location.
■■ Container size (if the material is stored in a container).
■■ Work center (or supplier) of origin.
■■
■■
QUANTITY
“Normal” EOQ/ROP Pattern
Small Lot Size Pattern (From setup reduction)
EXPOSURES TO STOCKOUTS
Figure 15.13 Exposure to stockouts.
TIME
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Production Flow
Work
Center
1
Center #1
“Raw” Material
Work
Center
2
Finished
Production
Center #2
“Raw” Material
Finished
Production
“Steady-State”—no demand and no production
PRODUCTION CARD
MOVE CARD
Figure 15.14 Kanban steady state.
How it works The following figures illustrate the use of what is often called a twocard kanban system. The two types of cards are a production card (authorizing production
of whatever part number is identified on the card in the quantity specified) and a withdrawal card (authorizing the movement of the identified material).
At the start of the process there is no movement, since all the cards are attached to full
containers, as shown in Figure 15.14. It is only when a card is unattached that activity is
allowed. In this way, the number of cards will clearly limit the inventory authorized to be
at any location.
At some point, a downstream process needs some of the parts produced by work center 2 (in its “Finished Production” stock). It takes a container of the material, leaving the
work center 2 production card with the center. This illustrates two additional rules of the
system—all material movement is in full containers (recall that the container lot size is
supposed to be very small) and kanban cards are associated with a work center. This initial
movement is illustrated in Figure 15.15.
External
Demand
Work
Center
1
Work
Center
2
“Free” Production Card—
authorizes production to replace
withdrawn container
PRODUCTION CARD
MOVE CARD
Figure 15.15 Kanban step one.
The unattached production card is the signal to start the work center 2 production to
replace the container that was taken. To do that work they need raw material, which is in
the containers in front of the work center with the move cards attached. When that material is used to replace the work center 2 finished material, the raw material container is
now empty and the associated move card is unattached, as shown in the Figure 15.16.
Work
Center
1
Work
Center
2
1. Production container replaced
2. “Free” production card placed on new container
3. Move card on raw material container removed
and is now “free”
PRODUCTION CARD
Figure 15.16 Kanban step two.
MOVE CARD
Lean Production
Work
Center
1
391
Work
Center
2
1. Full container from Center #1 moved to #2
2. Center #2’s move card attached
3. Center #1’s production card removed and
is now “free”
PRODUCTION CARD
MOVE CARD
Figure 15.17 Kanban step three.
Work
Center
1
Work
Center
2
1. Center #1 produces part to refill container
2. Production card attached to container
3. Material usage empties raw material container
and “frees” move card that was attached
PRODUCTION CARD
MOVE CARD
Figure 15.18 Kanban step four.
The unattached move card authorized movement of material to replace the material
that was used. That material is found in the finished goods section of work center 1. The
operator (or material handler) will now move the material and place the move card on the
container as proof of the authorization to move the material. Before doing so, however,
they must remove the production card that had first authorized its production. That represents another critical rule for kanban: every container with material must have one, but
only one, card attached. Therefore, when the move card is attached the production card
must be removed. That is illustrated in Figure 15.17.
Now, of course, there is an unattached production card for work center 1, allowing it
to produce, using some of the raw material for work center 1 and freeing a move card for
that material, as shown in Figure 15.18.
This process continues upstream even to the suppliers, who can also receive the kanban move cards as a signal for their next shipment to the facility.
Notice that there are no schedules with this system. Production and movement of
material are only authorized purely as a reaction to the utilization of material for production downstream. The production of the final product may be the customer taking material. In some facilities there is a final assembly schedule for customer orders. In those
facilities that may be the only formal schedule used.
Also note that the cards only circulate within and between work centers, as shown in
Figure 15.19.
Work
Center
1
Work
Center
2
Move Card Movement
Production Card Movement
PRODUCTION CARD
Figure 15.19 Kanban circulation.
MOVE CARD
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Chapter fifteen
The question is often raised about what the quantity or batch size of the kanban
should be. There are several rules of thumb around the logic. If containers are being
used, it should be a container’s worth. If there is no container, and it needs to be more
than one unit (which would eliminate all wasted inventory), then a standard quantity
should be set that keeps the work center running while another kanban is being produced; in other words, an amount that covers process lead time. Many believe that
a kanban should not be more than 10% of a day’s requirements. However, the more
important rule is that it be a disciplined quantity that keeps the flow moving at a level
pace.
Once the amount of the kanban has been determined, the calculation of the number of
kanban cards can be done. Again, there are several ways of computing this. The following
is one that is simple and easy to use, and shows the similarity to reorder point.
# of kanban cards = 1Demand During Lead Time2 >kanban quantity
Kanban “rules” Even though there are no formal schedules in a kanban system, there
is a fairly important set of rules or set of constraints under which the kanban system can
most effectively operate. Those rules are summarized here:
Every container with parts shall have one, but only one, kanban.
There will be no partial containers stored. Every container will be filled, empty, or
in the process of being filled or emptied. This rule makes inventory accounting easy.
Only containers need to be counted, instead of individual parts. The number of containers is then multiplied by the number of parts in a container.
■■ There will be no production or movement without an authorization in the form of an
unattached kanban card.
■■
■■
Card alternatives Since the development and successful implementation of kanban
systems in many facilities, many alternatives have been designed and implemented. Some
of the alternative methods include the following:
Single card systems. The single card is the production card, with the empty container
serving as the move signal.
■■ Color coding of containers.
■■ Designated storage spaces.
■■ Computer systems, often with bar coding serving as the signal generator.
■■ Workflow signals to suppliers to release product against a purchase order.
■■
It should be noted that the method used is not important. What is important is that
there is a clear reactive signal to generate activity that everyone clearly understands.
Using the Kanban System for Process Improvement
Because the kanban system allows for a controlled inventory of relatively small containers, there is a great opportunity for using the system to promote continual process
improvement. Specifically, whenever the process is working smoothly for an extended
period of time, there is a possibility that there actually is too much inventory in the system.
The analogy that is often used is a river. If the water level is high enough, it will cover all
the rocks in the river and appear to be running smoothly without any obstructions. The
water in the analogy is inventory, and the rocks are process problems, including quality
problems, worker skills, equipment breakdown, and so forth.
The approach is to gradually remove the water until the first “rock” is exposed,
thereby establishing a priority of the most important obstacle to work on. It would be
dangerous, of course, to remove too much “water” at a time, because the obstacles may
stop the flow altogether. This is where the small lot size of kanban is a benefit. Removal
of one kanban card will remove one container, and since the containers are small, so
too will be the impact of the removal. The important aspect of this is that some process
problem will ultimately emerge, signaling the next target for lean production process
improvement efforts.
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This is not an easy approach to implement. What is implied is that every time a
process is working smoothly, there may be too much inventory and what is needed
is to remove inventory until it “hurts.” That is certainly not a natural action for most
people, and the performance evaluation system needs to be altered to reflect this type
of activity.
Some Additional Lean Production Tools and Concepts
Value stream mapping Value stream mapping is a tool to map and understand
the flow of materials from supplier to customer, focusing on not only understanding
the current state of process and flow but also specifying the value-added and nonvalueadded time of all process steps. This includes all activities, even the in-process storage
of inventory and key metrics for the process. This visible understanding of the current
flow of activities allows for developing a future state map that will significantly reduce
waste, decrease flow time, and make the process flow more efficiently and effectively.
Strategies can then be developed for specific actions that will lead the operation toward
the future state.
Kaizen Kaizen is a philosophy of continual improvement that emphasizes
employee participation. The kaizen event usually focuses on a fairly small part of the
overall process to improve that part of the value stream. It is a structured approach to
understand and redesign the process to meet specific process goals that are often part
of the overall implementation of a lean production system. Kaizen events are generally considered to be a major part of the overall approach to continuous improvement
for an operation. Since kaizen events are generally designed to be accomplished in
short periods of time (one to two weeks), the term kaizen blitz is frequently used. An
example of a goal where a kaizen event might be used would be to reduce the changeover time on a machine from 16 to less than 8 hours. Another example might be
applying continuous improvement steps to be able to process customer orders within
4 hours of receipt.
Takt time Takt time is thought of as the heartbeat of the overall process. It is usually defined to be the rate of production that is synchronized with the rate of customer
demand. If the production process is well synchronized with demand, the implication is
that customer demand will be met with little or no excess inventory or other forms of
waste. Takt time is a common metric used to help design a future state process during
value stream mapping. Line balancing can be achieved by aligning cycle times and process times to takt time.
A formula for determining takt time is:
Time available during production period
Time
=
Unit demand during production period
Volume
example
Available work time
Shift 1: 6 am to 2:30 pm with 20 minute lunch and 10 minute break
8.5 hours * 60 minutes = 510 minutes
510 minutes - 20 minute lunch = 490 minutes
490 minutes - 10 minute break = 480 minutes
480 minutes * 60 seconds = 28,800 seconds
Customer demand
230,400 units annually
240 working days per year
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Chapter fifteen
230,400 units>240 days = 960 units per day
960 units per day>2 shifts = 480 units per shift
Takt time
28,800 seconds per shift
60 seconds
=
480 units per shift
takt time
5S approach While originally developed from a series of Japanese words, a rough
English translation is sort, straighten, shine, standardize, and sustain. In general, it is a
structured housekeeping approach to organizing the operation for more effectiveness and
less waste, which is the overall goal of a lean production system.
Sort. Determine things that are needed and those not needed in the workplace.
Remove those that are not needed.
■■ Straighten. Put the necessary things in good order so they may be readily available
when needed.
■■ Shine. Clean the area, and keep it clean.
■■ Standardize. Maintain the order and cleanliness that have been developed.
■■ Sustain. Train and develop attitudes to keep the orderliness as an expected and ongoing part of the organization culture.
■■
Some organizations have added a sixth S to the principle, and include safety, as good
housekeeping can be beneficial to providing a safe work environment.
Visual management Similar to the use of kanbans as a visual signal, visual management allows people to see what is happening and rapidly respond to issues as they occur.
Visual systems eliminate waste as the status of the process is very apparent, and no time
or energy is wasted trying to determine it. Many tools can be used to provide visual management, including charts, colors, lights, sound, or empty spaces. Display boards or charts
posted where they are visible can easily display key information. One tool, an andon, is a
signaling device within a process. In some cases, it works similar to a traffic signal, using
green, yellow, and red, to alert personnel of what action is required. Examples of everyday
andons include displays of customer wait times, the fuel warning light in an automobile, or
the walk signal at a crosswalk.
Lean accounting Accounting in a lean environment is much different than a traditional cost accounting structure. Typical cost accounting gathers detailed information
from each work center